Guess I have to disagree with a lot of people here on the wheel front at least. Most gravel bikes that are not top end do come with heavy wheels, but they are strong at least, and that's all you really need to start riding lots. A heavy second wheelset with no aero properties is also perfectly good for mud tires that are ready to go when needed, or to use with a wheel-on indoor trainer.

As for the rest of the bike when considering frameset vs. complete build, you have to consider how much you want to change, the price difference, and what you could recoup from take-off part sales. You may also want to keep some of the replaced parts as spares, or to use on other bikes.