whatever you buy, ensure it's all 100% perfect. i think a car with niggles - no matter how small - is a sign the car is not 'no expense spared', and may be only scratching the surface of more serious issues underneath. sure, you still can't be certain even if the car appears perfect, but it's a good starting point.
your 147 is an example - all those niggles probably mean the car has not had a perfect history. maybe it was sold cheaply, but that would be why.
if you buy a GT - or any alfa for that matter - worth paying a premium if it means buying into a top quality one. you may otherwise regret it (again, still no guarantee). maybe check out secondhand ones at a reputable dealership.
V6 3.2L is superb, so i too recommend it. diesel sounds like the more sensible version though, especially if petrol will cost you lots. except, as with any 3.2L V6 (6spd box), you MUST also budget for a Q2 upgrade to avoid future diff failure, as well as markedly improved performance. i suppose things start becoming expensive, but it's a bad idea IMO to not have the Q2.
good luck. but don't rush into anything. if the 147 is your first alfa, then perhaps keep it for a while after fixing everything, then plan your next upgrade and save up.