Constant torque would be the way to go, especially if you want to do tapping. Torque changes with speed/frequency on a variable torque unit.
As for your motor you should be able to double the speed with out much concern. Normally a motor can run between 20 to 120Hz and still be reliable.
With so many brands out there I can understand why you would be overwhelmed. Personally I like the AC Tech or Lesson brand VFD's. I have had good results with them in the past.