These are problem areas;
156 2.0 TS
Body/Trim - look out for rust bubbles at top of front screen and excessive stone chipping on front end, also poorly fitting side skirts
Interior - check carpets aren't wet on sunroof cars, check no strange airbag lights are on, make sure fuse box cover stays up, check carpets for splits on early cars. Make sure air con is working, air flow is crap though. Check boot surround isn't too badly damaged.
Mechanics - check for cam variator (diesely noise) on start up, check rear brakes haven't seized, listen for front ball joint clunks at the front, check oil level.
Spec - Silver or Black cars with Leather worth most, Sport Pack also good for resale, but leather is the big one. Avoid Achille Blu, Amazon Green and Africa Grey for resale, Proteo Red not good either - down to personal taste though.
Re 146 Ti
Body/Trim - check bumpers haven't faded too much and check door bottoms for v.slight corrosion
Interior - Quite robust, just check general condition
Mechanics -same as 156
Spec - you should get a later car than equivalent 156 for the same money, leather is nice but air con standard on facelift as are 5 hole alloys.
My Opinion: If you want a sportier drive go for the 146 otherwise take the 156, bear in mind you will probably be able to find a 2000/2001 146 still under Alfa warranty for £6-8000 whearas most 156s will be 1998/99 cars and out of warranty.