I've had two 147s, for 9 years in total, and have never had a problem with them. Best advice is to read the forum here, but remember that people usually only post when they've got a problem, so don't take the number of "problem" posts as meaning that every car has problems. Personally, my 147s have been the best-behaved cars I've ever owned.
A few things I've noticed with my two 147s:
1. Timing belts! The interval is very short - don't risk going over it, and don't buy a car unless it's got official paperwork to show a new belt was fitted. Intervals are every 50,000 km. Also, be aware that part of the variable valve timing system (inlet cam variator) wears, and needs to be replaced with the timing belt. The symptom of a worn variator is a diesel-like engine sound at startup and loss of engine pick up. If this is needed, it'll increase the cost of the belt service.
2. Suspension. Depending on which revision you get, the stock suspension is a little to bouncy (pre-2004), or a little too firm (post-2004). On mine, I invested in a set of Koni FSD shocks, and while they make the overall ride a little firmer than I'd like, they absorb road imperfections far better than the stock setup. On the stock suspension, the bushes often wear early, particularly on the rear suspension.
3. Suspension, again. The front wishbones can wear out - cheap enough to fix, though; the symptom is a squeaking sound when you go over speed-bumps. (Note, a similar squeaking can also be due to dust collecting on the rubber parts, so try hosing down first).
4. Oil: These engines take a lot of oil. Check it regularly. My first 147 (2002) consumed quite a bit (I'd need to add a litre every 8-10 weeks), but the second (2007) has been much more frugal.
5. Cruise Control: The electronics for the cruise are all in place in every car, you just need the control stalk. There are posts here about how to find and fit it. I would definitely recommend cruise control, btw: it's too easy to lose your licence without it.
6. Rattles: when closed, the rear hatch rests on a pair of springloaded rubber stoppers. These can sometimes make cracking or rattling sounds. The fix is again easy (they have screw-heads inside them that allows you to lock them closed, and this removes any rattling), but it may be something you can talk down a price with
Apart from that, there's very little I'd say against the 147. I know three other people who had them, and none of them had any problems with their cars either. That's a small sample, but it's a hell of a lot better than my VW-owning friends could say.