As said biggest cost is fuel really.
Diff mounts can go but not a diff unless you drive like a complete twat.
Heads dont go on 205's unless its got other issues that have caused it to go.
Generally very very reliable unless you buy a bag of shite.
BUT and its a big but these were £30K cars in 94 and just because they fall into the affordable to buy category you have to correctly maintain and service them with good quality products.
Lets not kid ourselves they are old cars now and other things will start to deteriorate as time goes by but thats like any other car.
Buy a good un and it will give plenty of miles of happy motoring - but if you cant afford a good un you deff cant afford a bad un