I'd suggest making a list of all the threads you need to cut, and figure out
how to make those parts on the lathe you intend to buy. For instance, I
needed to buy an extra 100 tooth gear to cut coarse threads on my
Sherline, while cutting metric threads (39x1mm) wasn't a problem, even
with a 20tpi lead screw. (127 tooth gear is standard :-) )

Zack Lau W1VT