About 3 months ago I bought a 1990 Rigid Steel Specialized Hardrock Sport. I have made it my project bike. It was in pretty rough shape, although the frame was in very good condition, just a few surface rust spots here and there. I have nearly stripped it of all its paint, and got some new decals for it, I am going to leave it raw steel finish and put lots of clear coat layers on it. Also I have decided to make it a single speed, for simplicity and just because I want to. My question(s) is/are:

What size rear cog should I use? I live in ky and will be trail riding, sometimes riding atv trails that are kinda steep. Also my choices for a chain ring are 28T, 38T, 48T.

Also what are good cheap tires?