No matter what way you swing it, building your own car will “almost” always cost more than buying a built one.
The only downside to buying someone else’s car is not knowing the PO driving style. Drivers can destroy cars faster than wrecks can sometimes. As an example, back when I raced Hondas there were a few guys that would complain to high heaven that the gearboxes in the Si were junk since they kept blowing out transmissions. The rest of us never really had a problem. Finally had a chance to watch in car video. Yep, coming into the braking zones, dude was just pushin the clutch in and ramming it into gear and popping the clutch back out. Knew lots of folks that would routinely over-rev the engines on downshift. Curb jumpers destroy suspensions pretty quickly too.
But, if you have the extra cash and time, by all means go for it. Personally, I chose to start with a very nice E30 (spent $3,500) for a 0 rust car that was perfectly straight.