Personally, while I like the A2 a lot, it feels to me like it's no longer the hidden gem it still was a few years ago. ?
The body won't rust and that feels like it's one of the few advantages to the A2 left these days. (other than design and interior space)
Parts are getting harder to come by, quite a few are no longer available new. If one of those goes bad, good luck finding a breaker. If you need one of them and there's none on Ebay-Kleinanzeigen, you may be without wheels until one shows up.
(Afaik, we do not have the luxury of club breakers like the UK has ? )
Is it still affordable car if you have to pay someone for repairs? > Depends on your budget. I recently paid over 500€ for new rear (disc) brakes. Not sure how that compares to a Polo etc. though.
What's an affordable price? Depends on engine, mileage and options. German prices seem slightly higher than UK prices most members here will be familar with. Personally I'd rather pay extra to have something special, others will be happy with a 1500€ silver car with no options.
The optimum engine will depend on how much you plan to use the car. The diesels are more expensive tax-wise (231€ 1.4TDI, 94€ 1.4i) so you'll have to catch up on that with cheaper fuel. I believe there were recent discussions regarding making diesel fuel yet more expensive so you may not be able to make up the difference anymore in the future.
(Funnily enough, in the UK it's the other way round where the diesels are dirt cheap tax-wise while petrol cars are taxed higher and fuel prices the other way round)
Newer 1.4TDI 75 ("BHC") > Older 1.4TDI 75 ("AMF") due to Euro 4 vs. Euro 3.
The different codes for the 1.4i (AUA vs. BBY) don't make a ton of difference. Both are Euro 4.
Probably avoid 1.6 and 1.4TDI 90 due to essential parts seemingly no longer being available new. Both are Euro 4.
Definitely avoid 1.2 for way too many things that will go wrong and cost a fortune to fix.
Mileage-wise, I believe that neither the 1.4TDI nor 1.4 have much trouble reaching 300k km.
1.4is will often burn oil way before that (mine did at 60k km...) and sound like a diesel, though that will usually not break them as quickly as you'd expect.
I'd look for a well-serviced example over one with low kms and no service history.
In my opinion, the interior can start getting annoying with cracks and rustles from about 150k km.
At about that mileage, they may also need new suspension.
Regarding Euro 3/4, while most A2s will be listed as Euro 4, I'd be suspicious of ones (EDIT: =Diesels) with a smooth service flap to be Euro 4.
AFAIK, the Euro 3 versions ("AMF") can be fitted with a particulate filter which makes them eligible to have a green Umweltplakette (Windshield sticker thingy to enter city centres), but that does *not* make them Euro 4. The latter "BHC" engine is Euro 4 from the factory.
If going for an "AMF" anyways, make sure any green stickers are legit, not bought off ebay. No filter, no green Plakette.
Right now (in germany), there's effectively no difference between Euro 3 with a filter and Euro 4. (Your mileage may vary, I am uncertain about places like Stuttgart and Hamburg where Diesels <Euro6 seem to have been banned on certain roads iirc) Who knows if or when that will change.
I believe Euro 3 is already a problem if you want to enter somewhere like London.
The A2 these days (in my opinion) has many of the same problems as other 20 year old cars.
1.4TDI BHC (04+) if you want or need a Diesel, 1.4i if you don't.
Service History over mileage. As few kilometres as you can afford (though you don't want a museum piece).
A german list of things to look out for can be found here: https://wiki.a2-freun.de/wiki/index.php/Checkliste_Gebrauchtwagenkauf
Beware that the list is a little old and a few extra things have surfaced in recent years like lower control arms rusting away.
Personally, I've had over half of the things listed there go wrong over 2 A2s and I love 'em anways. ?