To tell you the truth Michael I have yet to actually discover any rust on the three Alfa's I have owned, the one my father in law owns yet my brother in law's had some on the front floorpan.
It's subjective on how well they are looked after, I have to say that my brother in law somewhat abuses his example and treats it like a work horse really
We did discover the reason for the rust was down to the simplest of reasons.
He was a lorry driver who worked away all week so the car was standing for five or six days at a time. Combine that with the cold weather over the winter, sunny days and one main factor of an electric sunroof (something that I or my father in law don't have fitted).
The condensation that this caused actually soaked his carpets directly below!
Now as you may well know by now, the body is galvanised which I can only assume is only on the outside and not on the interior. Thus the rusty floorpan.
His is a 2002 2.4 10v 156 Lusso interior facelift model and that actually shocked me to think that rust appeared on a relatively newer version in comparison to our older 156's. I have to mention also that underneath there was little or no sign of the rust that was obvious, just small tell tale patches of underseal blistering!
The first thing that my father in law & I did was to lift our carpets up and give the floor and sills a thorough inspection inside and underneath..... Nothing, not even a spot of rust!
Needless to say, the first bright day we had we cleaned up and undersealed the floorpans. We also used a lot of self galvanising spray paint on the inner floor to prevent any future issues. It does mean taking the interior out and resetting the airbag light afterwards, but peace of mind is worth the couple of hours it takes.
I can only assume that condensation + sunroofs are a weak link in the galvanised armour and/or seriously wet carpets.
Avoid them if fitted if you are concerned, even take a screwdriver and ramps to check your potential buy. lift the carpets to check and above all, do your homework on regular faults and servivce requirements and you will have no worries that will hit you hard