stay away from the petrol options unless you can afford to run a 3.2 V6, however that will not make a great daily driver unless your mileage is below 15k a year...in fact for safety I would say 10k but you could push it...they are all older cars as the Busso was not an option in the UK after 06, most are 04 and 05 plates so you will in addition to all of the obvious stuff look for rust and expect to change suspension if it hasn't been done. it IS the best engine though for the complete GT package economy aside.
just remember it will be a 10 yr old car at least...exercise caution.
the 1.8 twin spark was only available for a short time in the GT and is underpowered by a good bit, lacks torque and economy will be terrible as you will have to rev the nuts of it to get anything resembling power...rated at 140 BHP feels like less.
JTS is terrible IMO, I have owned a JTS powered GT and never again will I go near an Alfa JTS engine, buy one at your peril. every specialist I have spoken to agrees its pants. 165BHP and is fun to drive but has some design flaws that can make it very expensive too fix if/when it goes wrong.
leaving the 1.9 diesel...150 BHP but 300nm of torque in stock form, very mappable, all the post 06 (CF4 engines) can be mapped to 210 BHP without to much trouble, as with all diesels of that generation emission controls (EGR, swirl flaps) are problematic and should be removed.
in short get the derv, have the swirl flaps and EGR taken out, re-map it, budget for a new clutch, a Q2 or clover 170 with a Q2 is your best starting point.
expect to do front upper and lower wishbones at anything over 50k....
almost Busso performance without the wonderful soundtrack and probably easier to live with day to day...
there is a nice one in the classifieds at 5K thats fits the bill, atlantico blue ,Q2 nd a 170 BHP. http://www.alfaowner.com/Forum/alfa-...-for-sale.html (Heads-up! Alfa Romeo GT Cloverleaf 170bhp Q2 for sale)
suspension map and flaps egr should be doable for £1500 in one hit...then you will have a cracking GT and better buying from an enthusiast on here than a dealer who just happens to have an Alfa, it will have likely been looked after properly.