You can do it with just a drill press, depending somewhat on the quality of the lower you get.
Most likely things like the mag well and such will be in need of work and you can either make a broach that fits in the drill press and broach it to dimension (maybe along with the rest of the inner dimensions, again depending on the quality of your lower) or spend a good deal of time with a long file or two and a large amount of patient care. This will probably require making a jig to hold it in exact position, but it can be done with a machine vise, some shimming materials and a bit of wizardry if you want to try that route (not the best idea for the first one). The inner dimensions are somewhat critical, but not in need of anything I'd call high precision machining.
Of much more need of precision -and what making the lower is really about- is drilling the holes in the receiver (for the internal parts) and tapping the buffer threads. Unless you are an accomplished machinist and can rely on yourself for absolute correct locating fo these drill holes, I'd suggest buying (or begging or maybe renting) a drilling jig that you can use to make precision locations of the drill holes. If they are off very much at all, the trigger group wont work and you'll have =at best- a highly unreliable rifle.
Cutting threads for the buffer tube is not difficult but either expensive or requires a large enough lathe that the swing will allow to single point the threads. You make a jig to hold it in the lathe by, essentially, clamping it to a proper sized square bar and mounting it tin the lathe with a four jaw (only way I've tried it, I imagine some ingenuity would allow you to use a round bar and three jaw). You can do it with a tap, of course, but they are a little hard to find and expensive and aren't a common size (1 3/16 x 16 with an odd thread shape IIRC).
Beena long time since I've messed around with that type of rifle -don't really like the AR's- but the regulations used to allow you to build anything you can legally own without permits or registration (don't even think about a short barrel or full auto) as long as it stays in the state it was made in and is not transferred. I'd suggest stamping the state of manufacture and a made up serial number on it, it has to be registered with the BATF as a manufactured weapon if it ever to be taken out of State or transferred (as in your will). You can. at least you used to be able to, have a machinist provide you with the machinery and instruction to make it in a machine shop as long as he does not do any of the work and only instructs you in how to do it and you are the one doing any physical handling of the parts or the machines (some shops used to have CNC programs for this and all you had to do was set the lower up in the machine under the owners guidance - check current regulations.
If you use aluminum, you also have to heat treat it. .There are a few materials, such as the ceramic/metal matrix and carbon fiber ones, that don't require heat treating but are sometimes difficult to find in an 80% finished version.
An AR is one of the more difficult guns to make, probably not the best to start with. Something like a MAC 10, 11, or 12 are far easier and don't require much in the way of precision or tooling at all to complete successfully. (again, check up on current state and fed regulations for the proper configurations for these to be legal).
