Darwin’s advice seems pretty good.
You want to avoid floors sold as “dance floors”, obviously. They jack up the price horrendously because it’s sold as a specialty. Try to find someone who knows what the floor is made of and make a similar one. Check out the floors of local studios and (slyly) ask where they got their floors.
Floors are priority, IMO. You can have bad mirrors and bad sound (you can even use a cheap boombox until you have money for something nicer), but a dancer needs to be comfortable on the floor they dance on. Treat the feet right.
What are the curtains for? Room separation?
Good luck! Please keep us informed on the project – I plan to do the same, one day.