Yeah BMW & Audis for under £1000 I'd be expecting a multitude of issues, no matter how good it looks. otherwise, well, why the hell is it so cheap? if they thought they could get more they would do so.
I got an 06 Audi A4 Cab back in May for £1500, looks fantastic, good MOT history but has high mileage (130k) and no service history, in the last 2 months i've had to buy...
new battery £130
'new' tyre - £25
about 4 different cans of contact/throttle cleaner - £20
new sets of bulbs for most lights along with lens repair stuff - £40 (after finding out a new set of OEM tail lights would skin
me £400)
special dead screw extracting drill bits to get rusty screws out - £10
new licence plate light bits and screws - £10
air filter - £20
1l engine oil top up - £10
Diagnostic kit - £35
...So already i'm on the best part of £300, and this is just for wee cheap things i'm fixing myself, i'm bracing myself for the service/electrical work/new 5 tyre set/transmission oil change at a garage that it really needs. (I'm likely looking at £50+ just for the super special unicorn piss multitronic trans fluid alone).
As a few others have noted, once something big goes on an older one of these, engine, transmission or turbo, it's v likely uneconomical to repair or replace, anything like that goes on my car i'll likely need to scrap/sell it.