i wish i had joined this forum and asked thesame question prior to me buying my 147. anyway make sure cam belts have been changed if mileage is more than 36k or car is older than 3 yrs old (whichever is most relevant). if not then u will have to do this yourself and it will cost around £400 or more i think.
one thing i would say from my experience is to find a 147 with low mileage or just expect to spend money on it afterwards. bought mine at 58k and since had to change all cam belts, variator, water pump and service it too. well i was stupid i didn't check service hostory b4 handing over my money then found out i had to service it at 60k
im not really a big fan of warranties but u might want to consider it but make sure u find out exactly what is covered and what isn't.
oh yeah one more thing make sure gear lever works perfectly i.e. it centers itself when u go out of a gear. alfa gears are known to get stiff thus don't return to neutral unless u push it there.
also check wheel alignment and make sure steering is straight when driving straight and check ride comfort too. nothing worse than bumpy ride.
as for 1.6 or 2.0, i would go for the 2.0 because i have a 1.6 and it is pretty fast but i was chasing a 2.0 one day and i saw the difference in power, trust me its quite a lot
. do a search on forum for more info on problems, goodluck