Shane, I'd be very cautious with a panel beater that takes your Alfa as is to repair rust. You'll see that the forum hasn't exactly been on board with suggestions as rust is a very individual condition to each vehicle. There's the obvios places to look for such as; front screen surround, A pillars, rocker or bottom door sill panels, floor, spare wheel well, front apron upper area where bonnet hinges in too and below, front wheel wells. A good way to find some hidden repairs and rust is to use a magnet and see where it won't grab on the body externally to find a panel of bog and/or rust.
It really comes down to a question of finance, as the best method to assess rust is to strip the vehicle to its shell and remove all paint. How this is accomplished depands on your finances, as with my prject I'm paint stripping one panel at a time and repairing as I go. If the tear down option is not possible make sure the body shop points out and shows you the rust they are quoting on. This way you can better evaluate the repair and trust me when i say that anything but new metal will not fix the problem, it will just postpone it.
Here's a few links to Alfetta rust repairs that may give you some insight. Strangly enough I came across my 78 Alfetta I bought off a forum member http://www.alfabb.com/bb/forums/car-restoration/9328-alfetta-rust-spots-cause-repair.html