What's your goal? Most solid drivers can coax speed out of a fast car built by somebody else. But the build experience has become, at least for me, a wonderful challenge.
If you're buying a proven car/bike, that might alleviate a lot of trips to the flats/airstrip sorting things out. But caveat emptor, mate - make sure you're comfortable with the vehicle. I suppose it's possible to buy a car that will get you a record and/or a hat. If you know the builder and are comfortable with their work, it'll put you in the game.
On the other hand, and I'm finding this out, there is a huge satisfaction in making every decision from the ground up and knowing every nut and bolt of the car.
If you want to go fast, buy an established vehicle.
If you want to learn how to be fast, build it yourself.