Thanks for the plug Jon.
Of course I'm no real expert but I've been riding solely single speed for a few years now so I'll have a go...
First of all welcome to the single speed world, long may you enjoy.
Nice collection of bikes BTW.
You're going to need an unramped chainring, probably best using a 32 tooth if you're going for a classic offroad 32:16 ratio. If you're a bit stronger or have a slower cadence try something like 34:16. It depends on your strength, terrain you ride etc but 32:16 is a good starting point. You'll also need some short chainring bolts too as the regular ones are a little longer. You can cut some down if you wish.
A decent chain is a must and also a conversion kit consisting of a 16 tooth cog and spacers to get the chain alignment correct, these fit straight onto your freehub.
Next you'll probably need to get some tension into the chain so a tensioning device will be necessary. You can get all this stuff off Charlie the bike monger on ebay just do a search for him on there and you'll see all the goodies he's got for sale.
If you find that you love single speeding after a while you might want to invest in an ENO Eccentric hub from White Industries, this will turn any frame into a single speed and looks really neat and tidy.
I think that's about it, if there's anything I can help with just give me a shout.