I've put plenty of work into plenty of vehicles, but mostly for the sake of keeping them in working order.
The most fun one was probably the 1980 Jeep CJ-5. When I lived in California, I could take the top and doors off in April and leave them off until November. The Jeep was already well set up for off-roading when I got it, so all I had to do was maintain it. It had an AMC 258 I-6 engine, T-98 manual transmission, and Dana 20 transfer case, among other features.
The best driver of the lot (before the BRZ) was a 2001 BMW 325i, and I did make some suspension changes on it. If I'm replacing blown shocks and struts, I might as well upgrade.
I also bought a used 2013 Sprinter. It needs a wee tidy up but is generally in pretty good nick so I've decided to convert it into a camper. I've figured out a layout and will be getting some bits done professionally but the
sprinter bed system is leaving me with a bit of a challenge.