Try the 2.4 JTDm especially if you can go for a facelift 20v car. Doesn't have the character or the power of the V6 but it is quick and the 5-pot sounds a lot better than some would have you believe. And you get 40-50 mpg.
It's also a lot more reliable than the twinspark cars, although I think the V6 is pretty good in that dept.
I've owned a 156 i.8ts veloce on an x plate which was pre facelift. i have driven the 2.0ts pre facelift on a couple of occasions depending on the year your looking at there is very little between those two engines as the 1.8 was 144bhp on a series 1 engine and the 2 litre was 150 bhp. alfa did down tune the 1.8 engine later to 140 bhp.
if your looking at a facelifted model i owned a 2.4 jtd 20v which was the 175 bhp version and also drove the 1.9 jtd.
if you want speed and economy with a i'd go for the 2.4 jtd 20v 175 bhp 0-62 is 8.2 seconds load of torque and the turbo kicks you in the back at around 1800 revs on standar diesel or a little lower on diesel plus. the cam belts only need to be changed at 72k on the diesel as they rev much lower than the petrol engine which need the cam belt changing at 36k expensive unless you go to an independant. still its cheaper than the belt snapping on you.
the 1.9 diesel is still a good engine but its not got the same grunt or 5 pot growl as the 2.4jtd 20v
dont listen to the v6 engine if you cant really afford to run it, i'm sure many will agree you can fall in love with the sound alone.
Have fun making your choice Sabratha just make sure whichever model you choose that you get a full service history and depending on mileage check if the cam belt has already been done.
Really, depending on how many miles you do, either the 1.8 TS, the 2.5 V6, or the 2.4 JTD. Even the 10v JTD is potent enough, especially after a remap, and would see off a 2.0 TS, and be on the heels of a V6. A remapped 20V would definitely see the 2.5 V6 off, and not be too far behind the GTA until you get up to ridiculous speeds.