If you're buying wheels, just go with a flip flop set up for rim or disc, depending on your preference. If you're able to run a single ring on your current cranks, they will work, but some MTB cranks would require using the small chainring BCD and you may not find a large enough chainring to make you happy. The spacer kit route will be a lot easier to work with if you're not experienced in dealing with chainline issues, such as respacing/redishing your rear wheel. You may need to run a tensioner unless you can find the "magic ratio", the perfect combo of freewheel and chainring to give you proper tension.