Sweet bikes the Avantis, great value for money.
Replace the chainring bolts quick smart though, they're only made of a soft alloy and shearing them would not be fun.
I think the wheel is a flip/flop wheel. So one side is designed for fixed the other for a freewheel (if there's thread on the non drive side of the hub, that's the freewheel side). A fixed cog is just a cog that screws directly onto the wheel with a lockring that is threaded opposite after it. That way the cog is sandwitched between the hub and the lockring preventing it from rotating backwards or forwards free from the wheel.
If you pull that cog and lockring off there will be a small amount of thread. You can actually put a freewheel over the top of that, which will give you singlespeed. Or you can put a freewheel on the side without anything on it at the moment, turn the wheel around and whallah you'll have a singlespeed. Easy!
And yes, that ebay link is the right thing. Any bikeshop will have dicta freewheels for $15, or shimano ones for $30 (get the shimano one). If you really want something bling, white industries make freewheels with 72 engagement points that are as loud as hope hubs.