I believe finishing an unfinished lower is a federal crime. You must apply for a serial number and wait for permission to proceed...iirc.
Finishing an unfinished (ie, less than “80% finished”) lower is like building your own firearm: you can do as many as you like, but you may not sell them without affixing a serial number to them. In some states, you’ll need a FFL to sell a firearm you’ve made. If you’re not selling or transferring a firearm you’ve made, the BATF is OK with you making your own guns from the ground up. That’s where the “unfinished lower” exception comes in. To the BATF, a compliant “unfinished lower” needs a lot more than a few holes drilled. You’re going to need a Bridgeport, a bunch of end mills, drills, setup blocks, etc.
For my money, if I were finishing an “unfinished” lower, I probably would not bother with someone’s forging - I’d just get a billet of 7075-T6 and mill it from start to finish. The hardest part of the whole business is the mag well, which you can either mill very carefully or (the better way) use a wire EDM.