cool that you're getting into mountain biking!
but you don't need to spent all the money you think.

singlespeed is great, simple, more reliable, but less versatile than gears.
you may want to try riding with the gears for a while first,
to make sure that you won't miss them if you take them off.
if you do decide to run singlespeed, a 2:1 ratio (chainring teeth to cog teeth) is a common starting point.
also you don't need to buy a new wheelset to run singlespeed;
you can just take the cassette off and run a single cog and a bunch of spacers.
but may need to get a vertical dropout chain tensioner like the surly singleator or rennen.

discs are great, all-weather functional, and can be real powerful.
but if your current brakes stop on a dime you can wait until you really think you need to upgrade.

have fun.