For under 5,000chf you can either get very lucky - or badly burned. Finding a fresk service/mfk vehicle is key - and then stick to the known high-KM cars. These are the ones that can be resold in Switzerland.
Generally the order for that is: Merc, Audi, Volvo, VW, Saab, BMW. I was told at the weekend they'd rather try to sell a 250k Saab than a 175k BMW
None of that takes into account age - but the same is true, stick to the solid/premium makes and things tend to run well. This means components are well made. Stuff can, and certainly will go wrong, but less frequently than on older makes - and a 15 year old car from one of the above is still a good piece of kit that will happily cross a continent.
Saying all that:
1) Avoid the Merc A-class it is woeful.
2) The old Merc 2.9 diesel found better use in the Sprinter. It is neither powerfull nor fuel efficient
3) 11l/100km is what a petrol Merc will drink (assuming mixed driving) - whether it is 4 or 6 cylinder!!
4) Do not buy a FWD Audi with the Multitronic gearbox.
5) High KM Audis litter the Export dealers - make sure when they say "with MFK" they mean freash one!!
6) Audi A2 is a bargain used - around 4-7k will get you the 1.4TDi or 1.6FSI model. A lot are well specced inside
7) The S60/V70/S80 D5 (163ps pre-particulate filter) is the one to have
8) VW 1.9 TDI engines are gruff and noisey - but arguably the most fuel efficient engine of the era
9) The 2.3 engine in the 9-5 is the pick of the bunch - Saab engineered through and through and no thirstier than the 2.0. The 3.0TiD has known oil level, cylinder liner and EGR issues.
10) The BMW M20 engines sound very nice (late 80s) but are moving into collectors status.
11) The M52B28 in 3er and 5er is one of BMWs absolutely best engines - expect around 9.5l/100km for mostly motorway driving.
12) The 3.0d is very reliable and infinitely smoother than the 25tds engine that came before it.