These Mr. Control single speed conversion kits will allow you to turn any geared bike (using deraillieurs) into a single speed. The sprocket will fit any 7 speed or 8/9/10 speed shimano type freehub body. The sprocket is made of steel and have tall teeth preventing derailment. I have run these myself and found them faultless. Spacers and lockring are included.
The sprocket suits 8 speed chains but narrower 9 or 10 speed chains will work too. I have runs 1/8" chains on this with no issue as well.
So if yo want a cheap simple drivetrain this is the best way to get it.