That's how I got into building my own. I bought a couple rifles and would replace most everything on the upper and lower i.e. grips, stock, handguards, trigger, optics, ...
It's much cheaper (especially if doing an 80%) and you won't have a bunch of leftover parts to sell or money spent on replacing those parts. Best thing is that you get exactly what you want right off the bat.
The minimum tools you need is a set of punches.