The Other Guy/ Paramaybe?
Thats what I did at first. Bought a completed lower and built that way. Now I have 2 80%'s sitting at home. One is already drilled out, I just have to mill it with my router.Probably start with a complete lower my first time around. But I do love the idea of starting with an 80% lower and enjoying the benefits of that.
Pro tip if you're going to go the 80% route. 80% arms has a jig that will allow you to do it with a hand drill, however after a couple holes my batteries died in my (cordless) drill and impact driver. I went to Harbor Freight and bought their drill press for like $60. Does the job fine. I would also recommend buying 80% arms router bit. The ones at the local hardware stored aren't the correct length meaning they don't go deep enough to properly mill the receiver.