Heads up, that bike and that converter are not going to be compatible. That bike uses a multi speed freewheel (7 speed freewheel from the description) and there is basically no way to make that bike's stock wheel work with any singlespeed sprocket, cog, singlespeed freewheel, ect...

You could buy that bike, that converter, and a new rim brake 650b cassette rear wheel that uses a cassette interface for about $65-90 (it has a free wheeling mechanism built into the hub and a splined interface lets splined cogs sit on it, where as a freewheel wheel has a threaded hub and a gear mechanism with a freewheeling mechanism built into the gears that thread onto the hub itself, not interchangeable)

Either get the converter and a higher end mountain bike with better wheels (usually by the time it has disc brakes it will typically have a cassette wheel), or get a decent but cheap Bikes Direct singlespeed mountin bike or a Nashbar singlespeed mountain bike (decent return policy) for around $400