Advice needed on purchasing a a3 Today

Sounds like a nice car.
The first thing I'd do is get a check done on it's history by HPI or Autocheck (who I prefer). I looked at 7 cars before I bought mine and had loads of things picked up by these checks. 4 had 'mileage discrepancies' and one was stolen recovered. I used the Car Reg Check text thing first to rule out the problem ones (it only costs £3 a go) and then if I went to see it I did a full Autocheck enquiry.
Also ask for the last MOT certificate number and put it in on the VOSA website and you can see all the recorded mileages from it's previous MOT's, along with all the advisories and if it had failed on anything.
I've not come across a model of car that had so many dodgy ones for sale. It seems they're far too easy to clock, or maybe just a sign of the times.

Secondly, the only big problem the early 2.0 tdi engines had was head related.
Have a good read of this and you'll know what cylinder head code to look for:
http://www.audi-sport.net/vb/new-a3-s3-sportback-8p-chassis/76277-%5B2-0-tdi%5D-how-know-if-your-cylinder-head-will-crack-if-hasnt-yet.html