Welcome to the forum mate.
The first thing you need to do is take everything you've ever been told about Alfa and throw it out the window. Because Alfa really aint all that bad.
Someone asked me today if my 156 (8 Years old now) had rusted away yet... Hmm, nope! Not yet!
The 156's were ALL galvanized except, I believe, for the roof, which in some cases may have some slight surface rust along the windscreen after about a decade or so old.
Truth of the matter is you are buying a fairly new 156, circa 2004 or whatever.
So, body wise you'll be fine.
Now then, mechanics? These are also fine, on both 156's I've had the mechanics and the electrics have ALL worked spot on.
Alfa used to have a pretty bad name for reliability and all that, but ever since the 156 / 147 era that's all changed, but no one except a select few have bothered to notice this.
Sure, if you look around the forum you'll find LOADS of threads with people moaning about issues with their 156, but that's what this place is: A forum. Where people come to find out how to sort out their motors.
You'll find way more 'I'm loving my 156!' threads by far I reckon.
They really are great motors, and if you find a well looked after example you'll be laughing.
Good luck mate, I really suggest one to you.