Finish sanding with a belt sander won't really work as is will create linear scratch patterns, unless you go up to a fairly high grit. However, it is a great way for fast stock removal. Be weary of creating valleys in the piece.
Most people won't use belt sanders as finish sanders because it's so easy to end up with a ruined piece in the blink of an eye. Look at this recent thread: http://www.woodtalkonline.com/topic/19613-whoa-times-have-changed-belt-sanders/
Yep, I'd say 50's from the retro design. That thing is cool, speed adjustment or not. You didn't say what speed it was stuck at,but imo, as long as it's not on high speed and you're using a large bit, you're ok.