If you put the rear derailleur on as a chain tensioner, you have already given up the whole of the low maintenance/high efficiency of a singlespeed, since the RD pulleys are what catches all the crap and need cleaning to keep the drive train efficiency at acceptable levels. If you want a few gears as simply as possible, a 1x5/6/7/8/9/10 set up is the way to go, i.e. one chainring plus a multiple freewheel. Depending on you spacing, it may be possible to put a 5-speed Shimano Cadet freewheel on your existing hub, then use a friction type gear lever on you down tube boss and you're ready to go 70s stylee
If you put the rear derailleur on as a chain tensioner, you have already given up the whole of the low maintenance/high efficiency of a singlespeed, since the RD pulleys are what catches all the crap and need cleaning to keep the drive train efficiency at acceptable levels. If you want a few gears as simply as possible, a 1x5/6/7/8/9/10 set up is the way to go, i.e. one chainring plus a multiple freewheel. Depending on you spacing, it may be possible to put a 5-speed Shimano Cadet freewheel on your existing hub, then use a friction type gear lever on you down tube boss and you're ready to go 70s stylee