As for singlespeeding a geared bike, the easiest/cheapest way (after buying one built, that is) is to remove the cogs you don't need and replace with spacers.
Remove shifters. Shorten chain. Leave derailler for tensioner.
If you really don't need the Spesh - sell that to fund the purchase of an off-the-shelf ss. Many threads on here about 'em. There's a few under £400.
Here's something for your speed/cadence/gearing choice: http://www.kreuzotter.de/english/ecadence.htm
As for singlespeeding a geared bike, the easiest/cheapest way (after buying one built, that is) is to remove the cogs you don't need and replace with spacers.
Remove shifters. Shorten chain. Leave derailler for tensioner.
If you really don't need the Spesh - sell that to fund the purchase of an off-the-shelf ss. Many threads on here about 'em. There's a few under £400.