Gear inches are gear inches. The only effect that you'd have to think about with regard to tooth count is whether it will help or hinder you in finding a magic gear if you decide to go that route.

Any 130 bcd chainring should work fine. I use an Excess Components freewheel on my single speed. It was $20 and gets the job done.

As far as tensioners are concerned, you could just use the rear derailleur that's already on there as a tenioner, assuming it's not busted. Note that if you're going to run fixed gear, you cannot use a tensioner.

As far as I'm concerned, the only downside to running any bike single speed is not being able to shift.

Valar morghulis.