Re: single speed bike converts to multi speed?
If you use the right kind of derailleur, you can easily mount gears on any bike. Use the one with the deraileur mount tab that slides in the axle slot. You might have to spread the rear tubes a bit for the wider axle, but it's easy with a small scissor jack or similar.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.