Hi and WELCOME TO THE FORUM! :wave:
There are two versions, the 8v and the 16v. As with any car, how reliable it will be depends on how well its been looked after by previous owners.
Common things to look out for include,
Electronic Power Steering failure. Check that the red steering wheel light doesn't come on whilst driving, and that the steering works smoothly in both normal and city mode. Causes for EPS failure range from simply a weak battery to failed steering control unit/ motor. Worst case scenario can be repaired for a couple of hundred quid these days.
Air bag warning lights. Check they come on and go out as they should. The two most common causes of air bag system problems are the passenger airbag isolator switch, and a dodgy earth, which wiring loom modification is available for.
The car SHOULD come with 2 blue keys (no red key). Another key can be obtained for around £100 if only 1 is available.
Timing belts. Check when it was last replaced. Is there a record of it? The 8v is a "safe" non interference engine, meaning if the cambelt breaks, there will be no internal damage to the engine. The 16v is NOT a non interference engine! Therefore arguably its more important to check the history of the camblet on the 16v.
Head Gaskets. Check for any signs that the coolant is being regularly topped up (ie weak mixture that looks like tap water) Also check for signs that coolant is mixing with oil or vice versa.
Leaky Sumps. Fairly common. The sump rots away and starts to leak. They are fairly cheap to replace. Prevention is better than cure though, so if its looking a bit rusty (but not leaking) give it a good coat with a decent metal paint to help prevent further degradation.
Parts are readily available and very cheap. Rust is starting to creep in on the older Mk2's so have a good check around underneath the sills, around seat belt mouting points etc..
All in all, if its been looked after well, they are pretty reliable cars
Im sure the others will add some more info for you!
Hope that helps,
Alan