Hi Sol,
I got an 1981 CB 650 Custom from a friend of a friend last year in much the same condition as the bike you describe. I think that the '80 650 and the '81 650C are fairly similar so I will comment on it. It took a little work to get going, but nothing major, and was pretty much a daily driver for 4 months in the summer. I think these bikes look great and are great to ride, and from my experience thus far, are fairly mechanically and electrically sound.
As for parts, there are a number of sites on the web where you can get new parts for most of the bike fairly cheaply, and Honda Dealers still have a fair number of parts for the bike. (However, I have found some parts, such as brake caliper housings to be difficult to come across). Also, this forum and its experienced posters are a great resource for information on repairs and parts. Since I've only had it for a year, I can't really comment on the durability of the engine other than it has worked fine so far. However, like any mechanical device which is 26 years old, its not a matter of if things will break down, but a matter of when, as you are probably well aware of with your 72 SL350. One thing I have heard is that the frame can rust out down near the center stand mounts so that may be one thing to look for.
Just like buying any used vehicle though, go over the bike a few times, go for a test drive, and if you like it and the price is right, my advice would be to go for it.
Rob
