You're talking about the car I nearly bought.
I bought my GT 3.2 V6 from Martin and Paul (Veloces) three months ago.
I nearly bought your one, but at the time it was up for £14k. And mine was an 04 plate on 28,000 miles at £13k. So I saved the grand.
Things you need to look at are:
Radiator, check for corrosion.
Upper and lower arm bushes, check for wear.
Test drive it again, and drive over some bumps and listen for any strange noises.
Check exhaust for rust.
Start the engine from cold.
Get them to do the cambelt. The market for V6 alfas is terrible right now, so do something about the price. But to be honest, if you can get them to do the cambelt included in the price and get their one year warranty then id say go for it.
Veloces are a reliable independant dealer. I've had dealings with them for ages and they've never really let me down.
My only problem with my GT was that the ECU was faulty immediately after purchase, but this got sorted under warranty.
Fuel wise you'll get between 20-25mpg with normal driving. Or looking at it another way, a full tank (£60) gets you around 250-300 miles depending how you drive.
Go for it. They are beautiful cars and in my opinion the last true Alfa with a beautiful Italian engine.