I've had mine for 3.5 years now with relatively few problems. They're not cheap to run though so I'm thinking of changing to something cheap and boring (the joys of home ownership). Here's a few things to look out for
The door seals can go resulting in water in the footwells. Happened to mine on the passanger side. However it was simply fixed by removing the trim from the door and squirting some silicon sealant in the gap. I've also heard it can happen if the pollen filter isn't fitted properly.
Coil pack failure is also pretty common but aren't too much to replace. I've never had that problem though.
Although not a common problem an issue that crops up quite a bit is the slave cylinder on the clutch failing. It's a gearbox-out job to fix. Mine is in the garage at the moment having this and the clutch replaced.
From reading around on the seat forums, people don't say a baffled sump is essential for track use but I had it done just to be safe.
The biggest gain on a standard car is getting it remapped. Expect around 260bhp. This puts more stress on the already weak diverter valve so make sure it's replaced with an aftermarket one. I have a turbosmart valve on mine which I can adjust to allow some air to go back into the intake and some into the atmosphere. If you don't allow enough air back into the intake it can start overfueling and run rough. In extrmeme cases go into limp mode.
Handling wise fitting a rear anti-roll bar makes a big improvement as it cuts out a lot of the understeer.
Also on the track I found fitting coilovers made a big improvement in cornering speed. I went for KW V1's although much firmer the ride over bumps and potholes is no worse than the standard setup.
Running costs aren't cheap. I average about 30mpg on super unleaded. Brake parts are expensive, I payed £250 for front discs and pads. Servicing costs aren't too bad with intervals at 10k miles. Also make sure that the cam belt has been replaced at 4 years or 40k miles. It says check at 60k in the handbook but they brought it forward.
There were 2 versions, 210 and 225bhp. All the mechanical bits are pretty much identical and remap to the same figures. But with £7k I'd expect a late 225.
There's only a couple of options availiable. Recaro seats and sat-nav (very rare). The recaro seats on the 210 were in the front and rear while the 225 was only in the front. I have them in mine and although looking pretty much identical in the front I found them much more comfortable.