If you would like to buy a GS/GSA, there are some ways to try.
You can consider a GS/GSA as you're daily car. But be aware that it is an old car and old cars break down now and then. Even if they're well looked after. A lot of people have a GS/GSA as they're second car, to drive in the summer, so that the car doesn't get affected by the bad weather in the winter. It should be advised to park the car in a dry, built over area, that way it wil stay longer in shape then parked outside.
First of all, many things can be said about construction quality of different types and ages of GS/GSAs. But since GS/GSAs are becoming rare it is better to look at individual cars then generalise whole series or types of GS/GSAs. A few considerations. Models anterior to 1973, they can be looked after for their historical interest, but often many youth problems. The GSAs built between 1984 and 1986 are generally not as good as GSAs built between 1979 and 1983. For the GS, most agreable are the Pallas, for luxury, and X/X2/X3 for their sporting look and performances (X2/X3). But then again X/X2 and X3 are more seldom than other models. Look after options as sliding roof, tinted windows, "Convertisseur/C-Matic" semi-auto transmission, vinyle roof (accessory) is very rare too, and the sliding rear seat can be useful if you want to use your GS daily. Avoid 1015cm engines, rare and interesting from historical view point but underpowered and often problematic. For the GSA, the X3 with its 5 speed gearbox is interesting. Although 5 speed transmissions where also available in other versions, the X1 and X3 have a more sporty transmission than the other versions. A GSA X3 in preference with options (alloy wheels, tinted windows, sliding roof) is very nice. A Pallas is nice too, again for it's luxury. Of course you can build options on you're Club, Special or other model, but beware that not all parts are easily available. Do not neglect limited editions (GS Basalte, GSA Tuner, GSA Chic and GSA Cottage Break) as they are very rare. Esspecially a GS Basalte is very worthwhile and can be considered as a full-option Pallas. Note that these limited editions have special parts, like striping, special seats, etc. If that has to be renewed, it will not be easy to find and certainly not be cheap.
Of course prices are dependable on a range of things, like the condition of the car, The country it is sold in, the type, the kilometers it has driven, the person or garage selling it, etcetera. The overall picture is that GSs cost more than GSAs and Breaks cost more than Berlines. Also mk1 GSs cost more than mk2 GSs. It is hard to give exact price indications, since this really depends on a lot of factors.
The 'street value' of a good GS is aproximately 2500 Euro. A GSA costs less, from 2500 Euro for a really good one to virtually nothing for a reasonable one. If you are going to restore a GS/GSA at a specialist, then 7500 to 10000 Euro is a good price indication. If you do the restoration yourself it costs aproximately 3000 to 6000 Euro. Usaully cars need a paint job (esspecially GS), and revision of engine and transmission.
Rust. A GS/GSA rusts, this is inevitable. The design of the car, with many hollow sections the short development time of the car, the poor quality of used steel, as in so many seventies manufactured French cars are the main reasons for this. Therefor carefully inspect a car at purchasing. The places to look for are:
Engine and transmission.
A GS/GSA leaks oil, this is also almost inevitable. Engines and transmissions that do not spill oil are extremly rare, or revised. This does not mean that the problem is important. Oil leakage can be considered normal for a car with an age of 15 to 20 years. Oil leakage can be stopped by usage of better materials or by revising the transmission or engine. Other than this, the 1222cc, 1129cc and 1299cc engines and gearbox, if they are properly maintained are very reliable and can easily cover 200.000km. Last but certainly not least, belts driving the camshafts wear should be replaced every 20.000km, this is very important! Make sure that the belts are on the right way, get them wrong and the pistons start hitting the valves and there goes you're engine. Further rubbers used in the engine have the same age as the car, and therefor they will brake now and then. replacing is usually relatively easily and they cost not too much. Also casket seals and other seals are usually of the same age as the car and therefor will be in need of replacing. If the engine makes a "clicking" noise while running, this may indicate a worn out camshaft. This is repairable, but expensive. A revision of the camshaft is a possibility and costs aproximately 500 Euro.
If properly maintained, this shouldn't give any problems. Check if the spheres have enough pressure, by putting the car in the highest level and look at the indicator underneath the hood. Start the car and see if it rises smoothly. Drive a bumpy road, if you feel any bumps then the spheres need replacing or need to be refilled. Spheres should be checked regularly. Check the hydraulic pipes underneath the car for rust. Pumps can leak but can be overhauled by specialists. Without special equipment this is almost impossible and the pump could be damaged. Too frequent clicking of the hydraulic pump indicates that the main hydraulic sphere needs to be renewed or refilled with LHM, which can be done by any Citroën garage. Renewing is better, since this sphere handles most of the pressure. In general can be said that the hydraulic system of GS/GSAs is very reliable. But the rubbers used, are most of the time as old as the car, and therefor there will be one in need replacing now and then.
Brakes and steering
Check the brake lines for rust. The discbrakes on the rear of the car can get stuck and therefor wear off unregulary and start to rust. Replacement is a considerable amount of work but parts are available. If the steer makes a noise by steering, then the steering rack is worn out and needs to be revised. Check also the rubbers of the steering rack.
Ignition and caburattor
A Weber caburattor is the best for the GS/GSA, the Weber are not very sensitive for dirt. A solex can be used aswell. For a 1222cc engine a Weber caburattor may be hard to find. Ignition, Ducelier or SEV. Ducelier is slightly harder to maintain, unless it is an electronily adjusted one. SEV is less reliable. You should not have many problems finding ignition parts, enough second hand available.
Tires and fuel usage
GS/GSAs are designed for the usage of Super fuel. However most GS/GSAs can use Euro 95 unleaded or 98 unleaded without problems. Except for the oldest GS models, who do not have hardened valve seats! Often the engine needs to be tuned by the change of fuel type. Use Michelin 145 SR 15 tires, these are specificly designed for Citroën and therefor they are the best choice. For the GSA alloys wheels, use the Michelin 145 HR 15 tires. If Michelins are not available you can use other brands.
Good bodywork is hard to get these days. And if you do find it, it may cost a lot. The steel used is not very thick, a dent is easily made. The interiour is not of very high quality, good seats and dashboards are rare. The electrical installation is reliable, however sometimes there may be a disturbance, which causes the most odd problems. Alternators are wekaer than average, but availability of replacements is good. Door- and contactlocks are weak an can be openend with a coin or a screwdriver.
GS Birotor specific
To cut things short, if you do not have loads of cash, do not buy this car. The Birotor is a really advanced example of Citroën engineering. But many times more complicated than a standard GS. So very hard maintainable and parts are extremly hard to find and extremly expensive. Remember that only 847 Birotors where build, of which most where destroyed. A Birotor in good shape will cost over 15000 Euro. The engine of the Birotor is not very reliable and is worn out after 60.000km. However the NSU RO 80 has the same engine as the Birotor, so this may give a lead to more spare parts.
Join a Citroën Club, it is not only fun to go to meetings and things like that.
But you can also learn from the experiences from other people. In clubs people
with the same cars are gathered, they can advise you on all sorts of aspects of
driving and owning a GS/GSA. Join a mailinglist on GS/GSA on the internet, this
works the same as with clubs, people share they're expierences and you can
benefit from that. Search a good garage. Nowadays there are not many garages
left who can service you GS/GSA in the right way. Therefor ask around with fellow
owners for a good garage, this can save a considerable amount of money, invested
in bad reparations. And in the worst case, this can even save you're car.