On CRC it will give you the option to buy a 203mm brake, which means it should come with everything to be 203mm.
For wheels, you can buy rims like spank rims or atomlab rims. Sun rims are also good.
For hubs, if you're going for rear freehub there's nothing stopping you using a pro II trials, seeing as you're going for singlespeed. If you want a screw-on hub, DMR revolvers are decent.
Ratio is something like 32:16.