I'm working on converting an old 91 stumpjumper to a singlespeed right now. I feel like converting to SS can be a pretty big range depending on your wants.

I lucked out that the frame has semi-horizontal dropouts. I could have done it just by getting the spacer kit and a rear cog, but I opted to update the brake levers (old ones were integrated with shifters) BB and crankset as well. Depending on what components are on it you might be able to pay for some or all of your conversion. This bike had Deore DX and some people really like that group - so I'm hoping I can pay for a lot of the changes by selling the vintage bits on ebay.