buying a a4 avant tdi sport.

You going for a 2.5 or 1.9? Check the arches for any rust if it's a later model as they're prone to it
 
1.9.
just seen the fault reader is discontinued
 
They lack some of the SE features and are alittle harder ride.A Quattro would be nice
 
VCDS is the only diagnostic reader I would use on a VAG car. Check for FSH or part SH. Make sure there are receipts for any work done not just a stamp in the book. Camblet, waterpump and tensioners are worth checking to see if recently replaced.

Check the arches for rust, feel if the car wonders.....can be £400+ for full new front suspension arms.
If manual, check for clutch bite feel on pedal, if high, counter in a price for a new clutch.

But like buying any car, give it a thorough check over. Gearbox etc, etc. There are buyers guides if you google search or search this forum.
 
Most A4's will bite high but do check for vibration on pull away as this could point towards flywheel and that coupled with a clutch will cost you around the £6-700 mark, feel under passenger seat for a dry floor as if this is wet it will point towards a blocked pendulum chamber and that leads to water getting into the CCM Which will result in you vehicle electrics having a mind of their own, the B6's are getting on a bit now so will obviously show it's age.
The 1.9 is generally a bomb proof engine and gearbox and you will often see ones with 200k + on the clock and still going strong, just be warned that if you will be requiring interior parts Audi have discontinued most of the trim prices now as I found out to my disgust so you will be relying on the second hand market.
Hope you find what you want and enjoy it they are great cars.
 
6 speed is a stronger box too, wish I had 6 speed. Clutch and turbo are the expensive bits, if the engine's rocking a fair bit the engine mounts have probably gone which will be £100+, you'll probably want to get rid of or clean the EGR too. Fairly solid in the engine department, I'd be more concerned with aesthetics and optional extra's.