The old missions from SB can actually be converted to SB ProPE. I did a bunch of them back awhile ago. It's not to hard to do but the biggest problem is the bridges. When you open the mission in the editor the bridges are missing. All you need to do is re-create them and save as a ProPE file.
My notebook has a GeForce GTX675M as a graphics chip and an i7-3720QM @ 2.6GHz. That's about as good as it got a year ago (as far as actual availability was concerned). Its performance is listed in the corresponding thread at SteelBeasts.com
(That is, assuming that we're talking about version 3.0, not the current 2.654; that one will run without trouble even on older machines.)