Fixed mountain bikes can be a lot of fun. I use one for winter commuting and trail riding (although I usually flip over to the coasty side for off-road stuff). A much cheaper option for converting your rear wheel would be to track down a cog that has been drilled to match a disk rotor bolt pattern (if you google tomi cog and london fixie I think you should find options). With the cog on the disk side, you can have a regular single speed cog on the cassette just in case.

For fixed off road riding, think of the lowest gear combo you could imagine riding on the street, then go one tooth lower. I have my fixed 29er set up at 32-20 but there aren't many hills to speak of where I am.

Also, if you're riding off road, PLEASE DO use brakes. At least one, two is better. Skidding all over someone's carefully groomed trail is a really bad way to impress the local MTBers.