I'm American (well, actually, I'm South African - but who's counting...) Any way, I did a simultaneous transaction where I purchased the Milano Verde from a Canadian and sold him my 164LS... (What a pain in the arse...)
Call me at 425 424 0100 and I'll give you the exact skinny, but in short, think American.
The seller should (read: must) send all of the paperwork to the Vehicle Export Agent (basically customs) at the border port of EXIT (think from our side...)
It's easier if the seller does it and he must do it AT LEAST 3 days before attempting to take the car there - (more if they are backed up) - They want to see the original title, the bill of sale and he'll need all of your info to take it there.
About 3 years ago, I imported a 1974 Alfa Spider from the US into Canada. Basically, if the car is more than fifteen years old, Canada Customs considers it to be a classic and the process is very simple. No duty is charged, but as usual, there's no escaping GST. I also had to pay provincial sales tax here in BC (but you have to do that on any new or used car purchase here). Not sure what the case is in Ontario.
If the car is less than fifteen years old, it gets very complicated. The government actually has lists of acceptable models which can be imported and those which cannot. For example, when I last checked it was OK to import a 1994-95 Alfa 164, but not a 1991-93. Check into this very carefully before trying to bring one into the country. You certainly don't want to buy one only to find out that you can't bring it home!
I'll try to find the appropriate gov't website for you.