I agree that you should probably pass on this.
That said, my first horse had stringhalt. She was also a bag of bones, had a big abscess on her face, and no hair. Naturally, I fell in love with her right away. (At $50, this condemmed horse was affordable...)
Anyway, eventually she had a tendon removed (I think that's what they did) from each back leg, and for the most part, she looked normal, unless she was really upset or excited. I jumped her, but she always popped her fences (very low fences.)
Just to say stringhalt doesn't mean a horse is necessarily useless.