The HP D530 CMT was a very solid computer. There were some BIOS issues in the beginning, but those were solved by HP. Overall, you can pick up a decent used D530 CMT for under $100 (no peripherals). It's a great way to get a solid desktop computer for general computing. The case is all metal, except for the face plate, and well put together.
However, the main catch is the PSU. It is a proprietary 240W unit, which has an unusual physical configuration. You cannot buy just any PSU to replace it, unless you wish to do some heavy duty modifications to your computer case. The OEM PSU has the switch and power socket on the same side as the exhaust fan. If you get a PSU with the exhaust fan on a different side, you're going to end up pumping air INTO the case, rather than out of it. That's a recipe for serious dust accumulation.
The other limitation is that the CPU can't be upgraded past a single core 3.2GHz. One other limitation is 4Gb of memory, which isn't too bad.
These days it's so cheap to get a new computer. However, the inexpensive ones will almost surely have a motherboard with many limitations. The better route is to buy a tower, motherboard, and CPU combination from a seller like NewEgg. They often run specials and you can secure yourself a very decent deal. The only other thing is to remember that there are bound to be tons of "little things" you need to buy that add up quickly... That's why a very good used computer is a great way to go.
Nobody has yet posted a really good writeup on a PSU upgrade for a D530 CMT. I hope to do one myself and post the results...