Figuring out what ratio you want: just ride it as-is and find the sweetspot. A gear you can live with all the time. On my SS MTB when I was commuting that was just over 2:1, but I've done as high as 3.1:1 on an SS road race bike. So on your bike, I'd think somewhere in the 2.2-2.4 range would be a good place to start.

All you'll really need is one of those generic "SS conversion kits" which usually include a single-speed chain tensioner (you can use your RD but that just looks hack IMO), spacers to fill in for the cassette, and an SS cog. Should be about $30-40 I think. You'll also need new grips after you dump those grip shifters. Single-speed chainring bolts to run a single ring, or keep the big ring on there as a chainguard if you want.

If you want to make it look the most elegant you'll find the "magic ratio" and run it without a chain tensioner