Hey. I've bought a 503cw +80mm one month ago, an enlarger, and have been spending way too much in the darkroom (=bathroom). And the photos look amazing, even with my lacking skills. No scanner yet to post stuff.
It's an awesome photographic experience. Diametrically opposite to the small format ricoh street shooting. It actually takes a decent amount of time and thought to fill the 12 frames.
Apparently the 80 mm is not regarded as highly as some of the other lenses, but it does have that 40mm equivalent (in 35mm terms) look that a lot of people, me included, like a lot. I'm not sure of the mathematics, but I don't quite agree that it's the equivalent of 50mm. It also has the "many famous photos were taken with this very lens" look
One thing I can warn you about is buying on ebay. I thought I got an excellent deal, until I took it to Hassy service for lube & adjust. There were various problems, and it cost a whole lot. So it could be good to buy from a reputable dealer for a little more, or be absolutely sure about the seller. I think you're also lucky, since one of the apparently most reputable service guys is in new york. David Odess, or something. He also has a very useful webpage.
I'd also search photo.net, they have a ton of information on medium format film.
I have no idea about the mamiya, but if there is a better experience than the hassy, I'd like to know.