A few other thoughts:
2020 T-slot framing is a lot cleaner and more flexible (er, in the utility sense, not in the wet-noodle sense) than the stuff from Home Depot. There's lots of accessories for it too, like angle brackets and connectors. Also, it cuts with a regular carbide-tip circular saw. Find it on the usual online places like eBay, Amazon, Banggood, etc; as well as 3D printer oriented sellers.
When laying out mounting plates, instead of circles for screw mounting points, make them slightly oblong slots. This allows for any mismatch or warping of the components to be taken up.
A flatbed scanner is a handy tool to have for getting mounting hole pattern dimensions off flat parts.
You'll definitely want a decent set of calipers - the inexpensive digital ones work quite nicely, but do be aware that they eat batteries like you wouldn't believe (they don't actual switch off - the only thing that the on/off button does is make the display go blank; the processor is still running keeping track of the encoder's position). These can be had for about $15-$20. Harbor Freight sells them locally, and they are available online in the usual spots too.
If you don't want to deal with batteries, you can get a set of dial calipers. These take a little bit of skill to read, but once you get the hang of it it's not bad. However, being mechanical devices, they aren't quite as robust as the digital ones (don't drop them), and super-cheapo ones tend to be kinda gritty and nasty to operate. If you go with dial calipers, there are ones available with both inch and metric reading scales on one tool; I would recommend these over the inch-only or mm-only variety.
The other measuring tool find myself using a lot is a Combination Square. Inexpensive ones (under $20, on sale sometimes under $10) will work just fine for this sort of project; you don't need to blow the budget on a super-nice Starrett set.
A miter saw is a good thing to have. I got one of the Evolution Rage 3 metal-cutting saws because it will also handle steel (the blade spins slower than a wood-cutting saw so it doesn't burn up the teeth on the steel that you're cutting) but if you aren't looking to cut thru steel then any inexpensive miter saw from Lowe's or Home Depot or any of the many online sellers (but, get a decent name brand one; they're not that much more $$$ but the quality level is usually noticeably higher) will work just fine. As mentioned above, with a carbide blade and a slow, steady approach, a wood-cutting saw will go thru aluminum. Probably obvious, but hearing and eye protection are a must (get several sets if you have interested bystanders); and watch your fingers! A dedicated vacuum cleaner is a good idea too (again, can be cheap, old, 2nd hand, or somewhat small, and it doesn't need to be a shop-vac... it just needs to suck).
You can get by with hand sanding if you want; but if you have a random orbital sander it helps take the tedium out of the initial heavy sanding passes for painting. A respirator rated for dry particles is a must. The same N95 or KN95 masks you'd use for COVID protection will work here, but be sure they fit your face properly. However, the "non-medical" disposable masks that are just a piece of paper for catching spray from your mouth and nose won't work against sanding dust, even though they do help somewhat with preventing you from accidentally spreading COVID if you have an asymptomatic infection. Nuisance dust masks like the kind that you'd wear while mowing the lawn are also not good enough for sanding dust, especially as you get to the finer grits of sandpaper.
You'll want a set of small hand files to get burrs off of cut/drilled parts.
A drill press is a nice thing to have, but it's totally not a requirement for this. A basic cordless hand drill, however, is. It doesn't have to be fancy, but it should have a clutch so you don't strip screws. You can still get NiCd or NiMh battery powered versions at the low end of the price scale, but I would really suggest stepping up one level to an inexpensive lithium powered one - they just work so much better. You'll want a set of bits and screw driver tips to go with it, of course. I'd also get a hand driver handle that takes the same 1/4" hex driver bits - sometimes a gentler hand touch is necessary.
Stainless steel metric button-head hex drive screws are available in bulk on the usual places (eBay, Amazon, etc) and both work great and look great also - and they fit the other 2020 hardware, too. Be sure your driver bit set includes sizes that fit any such screws you get.
If you don't have one already, get an electric heat gun. It's useful for everything from helping to dry paint, to bending acrylic, to shrinking heat-shrink tubing without burning it (you sometimes see people using a lighter or butane torch to do this, and it almost always results in the outer layer of the heat shrink getting burned).
Grab a pack of Sharpie markers with the wide tip; they make for a great scribing medium on aluminum. You cover the general area with ink, then use a fine tool like the tips of your calipers, a carbide scribe, or even a razor blade to make a fine mark where your dimension line should be. After you're done, it cleans up with isopropyl rubbing alcohol (and I suspect denatured alcohol from the paint section of your local hardware/home improvement store would work also, but I've never tried it). Masking tape and a very sharp pencil work nicely on other surfaces that can't handle the Sharpie ink.
A small prick punch, a small hammer, and an Automatic Center Punch are also important to have when laying out hand drilled holes. Start with the prick punch (it should be sharp enough to slide down into the groove of your scribe lines) to establish the location of the hole; expand the divot with the Automatic Center Punch, and then drill your pilot hole. Again, these are inexpensive tools available at the usual locations.