The pounds value gives us a clue you are in the UK. So over to the locals for their experiences. In Australia, a much warmer drier climate, our main concerns are, EGR valves misbehaving more of an annoyance than anything leads to poor performance, fuel economy issues. Next would be suspension rubbers which seem to 'go loose' after about 40,000klm causing all sorts of rattles and clunks. If it's an auto you are looking at, well they have there own website almost.
Honestly the major MAJOR problem we have here is poor dealer knowledge and servicing, they just don't seem to have the interest or ability to fault find anything that's not spelt out on a code reader.

 

Hi Alan
If its pre facelift (up to May 2010 approx), I dont think it has a DPF. Fitted to the later i30's and I havent heard report of a DPF issue. Mine doesnt let on its there except when the consumption gets a little wore for a few miles.
Lots of things to like about an i30, but my experience is far from rosy with lots of faults (albeit mostly sorted under warrany) and a car that creaks and rattles (likely because of radio and instrument cluster replacements, as well as the dreaded mirror).
Check out the trade value on Parkers etc - as I think UK residuals are woeful - OK if buying though.
Stick my neck out here & say that I think an Astra, Focus or Golf is a much better long term bet.