OK, is this road or mountain singlespeed? Are you going for fixed gear or simple singlespeed? There are many ways to do singlespeed conversions on the cheap. You don't even need to buy a chain tensioner on a frame with vertical dropouts if you have access to an old derailleur that you can use as a tensioner. Search around in the singlespeed and fixed forum here and read that sheldon brown website. Sheldon's site in particular is a great resource and will get you headed in the right direction. Got me started on my first SS, then my roadie to fixed conversion and my second SS (which is now an 8 speed, but still, the site helps a ton).