I have not played with swapping motherboards or power supplies myself. I think some boards have the same footprint and you might be able to swap them in the same case with the same power supply, but personally I've always at least gotten a bare bones package with the motherboard, power supply, and case. I know there are some different types of power cable connectors but I think there are adapters for that when necessary. So you might have to read up on that yourself.

Personally, I got my current computer from https://www.cyberpowerpc.com/  But today and tomorrow is a good time to buy whatever, whether components or barebones kits or PCs. Shop around.