If you're gonna get a 147, in petrol its gotta be a 2.0ts unless you can stretch to the big boy.
Ive found the petrol stuff more and more strangled by emission control as they have "progressed", and you will likely feel the same as i did, when i had a 2.0Ti 146, then the same lump in a 156, admittedly heavier but the motor just didn't feel as punchy, even when lightly modded, and i reckon you're going to experience the same coming from your 145 cloverleaf.
as for the diesel stuff, its brilliant.
ive now had alfa's in almost all shapes and sizes, and every engine bar a 1.6, but now run a 1.9jtdm 16v 6 speed Sportwagon most of the time, but still have a modified 2.4jtd 10v, and a 2.5 V6.
in the 147, youve just got to try the jtd stuff, particularly the jtdm 16v versions, they are brilliant!
coming from a modified 2.4jtd, i was concerned the 1.9 would be missing some power, but no it doesn't, and it is so free and worthwhile to rev, it is unlike anything diesel ive driven before, fantastic, and in the smaller lighter 147 body, i reckon it'd be brilliant.
so my two penneth would be a 147 1.9 jtdm 16v, decat and remap the thing to about 175bhp and gobs more torque, and you'll have a hoot!
cheers, happy shopping
ps - if you still want a Twinnie, my mother in law will have her 147 51 plate 2.0ts for sale soon, pretty cheap too i should think as it sits there doing nothing and needs to get used.