Common mechanical problems, are:
Variators, Rattle sound on start up.
Cam belts, Nothing to see or hear, but check if it's been done, if not get it done before you buy.
Cam Belt tensioner, makes a slight wine, and can ruin the belt.
Squeeky suspension, seems to be Bushes at various points.
Loose feel to the rear suspension, same cause as above.
Other than the above it is just like any other Car, Exhaust, Tyres, accident damage etc.
At that mileage and age a lot of the 156's you will find will be private imports, so check it out. The spec may be differant to a UK car and it may effect resale values.
Other than the documentation the easiest way to quickly tell is to check if the Chassis number is etched on to the windscreen. If it isn't then it is likely to be an import. (But it could have had a new Screen wink )
If and when you find one, post details on AO, someone will give you some advise on what to look for in the way the proper spec etc.