The first major problem is that it's not an Aero.
If you're after a soft ride, I've got an Aero and it's firmer than an Alfa 147. In fact, it's firmer than most things. As others have said, the acceleration is deceptive. Torque is limited in 1st and 2nd, so 3rd is as fast as second, which is surreal.
The rust under the tonneau cover is fairly common - muck accumulates under the rubber seal and stays there. Some got fixed under warranty. I'd say £150-£200 a side to fix it, depending on the colour. I know that as I've just had mine done...
TBH there's not a massive list of things to look out for on these - the key thing is that the roof goes up and down properly.
The right NGK spark plugs should always be fitted, or they can run poorly as the plugs are used as part of the knock detection system.
2003 cars had an SAI pump which pushed air into the exhaust. This can act up and be expensive to fix.
There have been some reports of intake valves failing, mainly on 07 B207R (Aero) cars, which leads to poor cold starting / running. These issues seem to be centred more around american cars.
Get one with the top of the line audio system. The audio system on the pre-facelift cars can't be replaced with an aftermarket unit easily, and the basic one sounds nasty. The better audio systems sound OK.
Oil should be 0W40 Mobil 1 or 5W30 full synthetic. Mobil 1 is the better of the two. Around £50 worth of Mobil 1 oil for a service if you buy it at the right price.
All the turbo engines (1.8t, 2.0t) are more or less the same - 2 litre turbo B207s with various maps. You can have these mapped to near Aero performance. Only the B207R in the Aero is much different, with a bigger turbo, different cam profiles etc. and will map up to around 250 brake. There is a 1.8i which is some GM engine and not particularly popular.
The diesels are the same as used in various Fiats and GM vehicles, and there a few problems documented I think. I know very little about them as I'm not interested in them.
The 6 speed manual box is also a GM unit, and like the 6 speed in the Vectra, it can be a bit crunch if you rush first to second. Apparently a gearbox oil change and adjustment of the linkage can improve things.
Oh, and the hood is around £800 to replace. You can reproof and recolour hoods, but if it's actually damaged, don't take any flannel about how you can get it replaced for cheap.
All in though, they're quick and very cheap - to get similar performance you'd be looking at a 328 or 330 e46, and a) they're a lot dearer, a grand or two dearer, b) the larger engined E46s tear their boot floors to pieces, which is a massive repair and c) older BMs tend to have 6 suspect previous owners...