Buy a complete upper and a complete lower separately. It should save you a little money.
My advice has always been "buy your first, build the rest".
I bought my first as a factory gun and then started on the accessories and mods.
With my current project I'm building the lower (SBR) and then will most likely buy a (mostly) complete barreled upper.
After that I may get brave and build the entire thing from the ground up.
I agree with this. You can buy the lower half locally at FFL and the upper through the internut.
Do not build your first. Get used to how they operate and work and you will appreciate your future build more later.
Right now is a good time to buy a used complete rifle, seems to be alot of them running around.