I just converted a cheap MTB 18 speed with an SRAM crankset (48t) and a Shimano single cog freewheel (18t). Mine is used for urban riding. I was lucky to have horizontal dropouts so a tensioner was not needed.

There is lots of info here and elsewhere on the web on disc brakes. I have discs on my other bike, but I have not messed with them yet while the bike is still under warranty :-)