If you are fairly knowledgeable you should really build one yourself. You can find tutorials on youtube on how to put together a computer and if you need help with components you can either look through the forums for similar budget rigs or I could help you put together a part list. I enjoy computers so I'd be happy to give you any advice. Also I'm new here to the forum so I'm trying to establish myself on here. Right now I have a pretty small mining rig at about 1 GHash but I'm looking to expand in the near future. PM me if you have any questions.
Know OP doesn't really want this answer, but I agree. Assembling a PC is simpler than assembling most furniture. Once you look at the motherboard and the parts with the cables they came with, it's fairly self-explanatory to figure out where everything goes. PCB's fairly flexible, and now almost everyone's stopped using a bunch of long pins on hardware, there's very little risk of messing something up. You don't even have to go through the trouble of trying to stuff all the cables in a metal box if you don't want - usually get better airflow with it outside a box.
