First. Does your mountain bike have horizontal drop outs? If so, then you probably won't need a chain tensioner.
second, are you thinking of replacing your hub? or getting a freewheel
Freewheel goes onto the hub
Here's the cheapest method:
Take chain, brake chain, shorten to fit gearing you wish from the already installed freewheel on you bike. Attach chain. Everything is left alone, you just can't change gears.
replace freewheel with MBX freewheel. You can use the rear deraillier as a tensioner - if you need it
Third, total fix:
Bmx freewheel, chain tensioner - if needed, 1/8 chain, tappered crank and remove all but one chain ring. Cut or replace tappered bolts to keep chainring on.
It really depends if you need a chain tensioner or not.