The seatpost is 27.2mm, that's a pretty common modern standard for aluminum frames.
You can convert the mtn bike hub to single speed, there are 2 different kinds, freewheel and cassette. Which have you got? The process is different depending what you've got, and it does require a special tool to remove the clusters usually, unless you want to get creative.
I'm running a coaster on mine, you can space the axle out to 135mm if you have enough axle. There are 2 methods for this as well, you either add spacers inside or outside the locknuts. You just need 12.5 mm of spacers on each side.
Inside locknuts (preferred):