The top five wheels in the $500-600 range that have the most versatility are:
Lendrum Original--single or double treadle, ratios from 5 to 44, folds up for storage
Kromski Symphony--double treadle, scotch or double drive, ratios from 5 to 25
Ashford Traveller--double treadle, single or double drive, ratios from 3.7 to 30, small
Ashford Traditional--single treadle, single or double drive, ratios from 3.7 to 30
Fricke S160--double treadle, ratios from 5.7 to 19.4, folding version, very inexpensive
So over the next few months, I will be looking at these wheels used and around the end of February I will be ready to spend money on a wheel. I'm very excited. I have all sorts of fiber sitting in my bins waiting for my wheel. Including a rainbow of corridale.