The Optiplex 745 MT is one of the few Optipods that can run a C2X chip, and actually overclock it. It has native support for 130W CPUs and good cooling. HDD are dirt cheap now. Obviously the guy's not made out of money or he wouldn't be asking about a 10 year old Dell. The cooler I listed can be had for $10-$15, the big fan about the same. he could start with CPU,RAM and HDD for about $100. He has the GTX750 already and it doesn't require a new PSU to run.
I actually overclocked one of these myself. it went 3.45GHz easily, but I threw in a 2motor fan setup I had lying around at the start so I don't know if it will do that speed or not with a more moderate setup. The E520 will go that speed without serious mods. The 745 Might also...