I agree with the ITX route.
I have a huge Thermaltake Mozart TX case and that can take a second PC, but ITX based, and so I decided to plonk one in just for a giggle, and while I was doing it, I bought a tiny little case just to put everything in ( knowing that I would transfer it later on ) the ITX setup that I did knock up, was a Core2Duo based jobbie, and the entire thing, I knocked up with buying the Mobo and CPU used, the HDs I already had, however, I ended up buiyng both a 120GB SSD and a 1TB LaptopDrive, and the case new, but the rest I had lying about, and in the end, I kept it in that little case ( its still in there ) and in my bored moments, I tried OSX and as it just so happens, it runs, and is running Tosemite 10.10.6 absolutely lovely, so its staying as-is.
My most recent build, again, with the intention of filling the Thermaltakes second slot, has also turned into a different PC than I intended it to, purely because its turned out to be a better system than I expected it to be, and it has now replaced my I5 as my Linux PC... Thats an AMD A10-6800K based thing, that again, I bought second hand along with a Gigabyte ITX Mobo and the case a coolermaster from the OCUK here MM for only 20 notes and its immaculate, the PSU is only a basic cheapie, again, SSD and 2GB but again, they were already here, the RAM is DDR 3 and so thats cheap enough to not worry about and there we go! - a fairly quick half decent setup that was knocked up at under a ton... Had I got a new HD, it would have still been under 150.
Ok, not brand new, but why buy new when there is often a great choice in the used market?