Get a variable speed. My first one (40+ years ago) was a single speed... 30K rpm. It just goes too fast for some jobs, so it isn't a high use tool for me. I still have it and still use it occasionally.
I picked up a Foredom at a garage sale with a foot pedal speed control. The variable speed and flex shaft make it very useful and I use it a lot.
Bottom line: at least get a variable speed. If in your budget, a flex shaft is nice to have too.
Ric