For singlespeed, chain tension isn't as big a deal. I think you'll be OK. Get a cassette hub and a couple of used cassettes. That way you'll have a number of cogs to choose from and plenty of spacers. Try and find a magic gear from those. The chain will become slightly more slack after a year or so, but it shouldn't be a big deal. If you had a couple chainrings to try (was your crank originally a double?) that would help. I'd start with the small ring and the 13T or 14T cog.