I bought a Porsche 924 off eBay for £160 and turned it into a "race car". I use the inverted commas deliberately. I sold some spare parts that came with it to help finance the build and sold the whole lot for £850 after we'd killed it at Snetterton.
Would I do it again? Never. I'd buy a ready made one it's far cheaper financially and emotionally. Having said that I've got a daily driver that I'm using for hill climbs, and Lord knows what'll happen to that