I bought a honda shadow 750 after taking the course. I was bored after 3 months. Now ive got an m90 and couldnt be happier.I just completed an MSF course in Kansas City. We rode a bunch of donated 250's. At the end of the course both of my instructors (two motorcycle police officers) said to get something bigger than a 250. They said we'd get bored with them in a month.
The class was awesome, but there's no way to practice afterwards. My girlfriend bought an S40 (single cylinder 650cc) which was a great transition bike for her and good for me to practice on, but not nearly big enough for me. I bought a C50 (805cc).
I don't have an answer for you, but absolutely find a class, and then go sit on some used bikes at dealerships. Don't waste time though. It can be intimidating, but you have to get right back on a bike and ride.
Look for something older, and you can get a pretty good ride for cheap. My instructor said, "The first bike you buy won't be the last bike you buy." Just find something you're comfortable with.