1) Conversions are not always economical.
2) Conversions are nice when you ALREADY have a bike and like to switch it up. It makes no sense whatsoever to have cash, buy a geared bike, then convert it. Just buy a used SS.

Just because it's on CL and it's old doesn't make it a good deal. $300 is too much for that bike. I'd pass altogether. With $300 + the money you'd spend converting it you could have a NEW SS MTB.

I'd hold out and save up for a used Bianchi S.A.S.S or P.U.S.S or something similar. One of those (can't remember which) was $750 new, so used would be close to $300.

There are LOTS of single speed MTBs out there. The SS MTB thing has been around longer than SS road bikes.