You may want to have a read of this:
http://www.seatcupra.net/knowledge_base/knowledge_base/seat_leon_faq.html
Standard issues would appear to be (feel free to correct me if I'm wrong):
1.) MAF sensor. You can check if its on its way out by getting the car up to 3rd/4th gear and up to 4k rpm. Rev it up to 5krpm (with your foot down as doing it gently doesn't seem to show it), and if it stutters its quite likely the MAF isn't working properly. This could also be the wastegate though.
£50-60 to replace the sensor - 2 minutes to do the job yourself.
2) Door seals. Water can easily leak into just about any of the footwells - but its usually the driver's to go first. Could ask if its doing it, or take a 2 litre bottle and pour water on the door to check. Could be the seals, the holes at the bottom of the door bunged up with wax and dirt, or the pollen filter if on the passenger side (as has been mentioned).
3) Window regulators. Check all the windows to see if they operate properly. If one makes a grinding noise, or doesn't return to the closed position, the rplastic regulator clips are broken, or the cable has snapped. £50 for parts, 1-3 hours to repair it yourself (there is a
guide here:
http://forums.vwvortex.com/zerothread?id=1015107 ).
There used to be coil pack issues, not sure how relevant this is now. You'll obviously be able to tell how decent the gear change is, but 1st to 2nd can be notchy until warm. Not sure if there's a conclusion to solve this
problem (any offers?). Some have had suspension issues too. Plastic water pumps can be rubbish too as mentioned. Oh, and models with blue paint stone-chip easily.
You should probably read this too...:
http://www.seatcupra.net/forums/showthread.php?t=33108
With it only having 23k miles on it I'd expect it not to have many issues...but you never know.
Incidentally, if you're thinking of stretching to £8-8.5k, why not stretch to the R model? It'll obviously be higher mileage for that price, but wouldn't it be worth it? The 1.8T is pretty bullet proof so I don't think high mileage is that big a deal (perhaps a matter of opinion though I suppose). Just a thought. I'm happy with my 20vT...