>>1232188IGH probably wouldn't be fun. I think they waste a fair amount of your pedal power unless they're something expensive like a rolhoff
You can convert ss to geared though, two options: derailer in the back, or chain tensioner + double crankset. Without a derailleur hanger in the back there will be some technical challenge
Front derailer would probably be cheaper because you wouldn't have to replace the rear wheel. Especially cheap if your front crank will take two chainrings
