That's the spirit! Just be careful, they're like potato chips and tattoos... Once you do one you won't be able to stop
As far as the mill goes, I would say there are 3 main factors:
1) Interest level: The cliche above notwithstanding, do you plan to just do this one lower, and possibly never do any metalwork again? If so, you may be better off finding someone with a mill that will let you use theirs; this may have the added benefit of some guidance/coaching from the owner if you need it.
2) Shop space available: Assuming you decide to buy a mill, many folks think you should get as much mill as you can manage. This may be limited by the amount of space you have in your shop/garage/basement/whatever, what the access to that space is like (stairs? narrow doors or tight corners? etc.). If you have plenty of space and can deal with getting it in there, a Bridgeport or similar knee mill would be great. They are huge, and heavy, but aside from the hassle of moving it those are very desirable qualities in a mill
You may be able to find a used Bridgeport on craig's list or somewhere for less than $1000. If you have less space or access issues, a benchtop type mill may be a better choice. The Industrial Hobbies
square column mill is pretty nice but not cheap (~$2400 + shipping). It's about as close as you can get to a Bridgeport on a bench. Yes, that's a HF 44991 mini-mill sitting on the bed of the IH in that pic
If that's still too big, the Sieg X3 or X2 clones from Harbor Freight
, Grizzly, or similar are the next step down. They run about $800-1000 for the X3 clone (HF 93885
) or $400-500 for the X2 (HF 44991
). These are Chinese machines with everything that comes along with such erm, "workmanship"
Not the most precision pieces of machinery but they can be quite serviceable. The X2 is the smallest you should consider; the micro-mill is just too small for the work you need to do. Also, square-column machines are generally preferable to round-column mill-drills.
Along with shop space, AC power can be a consideration. Depending on the mill, you may need just 110V, or 220V, or 3-phase, etc. Be sure you can give your chosen mill the juice it needs to run.
3) Funding: Guess I already covered that in the Space stuff. But you should also consider the tooling you will need: A vise, clamping set, cutting tools (end mills, drill bits, etc.), holders (collets, end mill holders) precision measuring gear (calipers, dial indicators, etc.), parallels, 1-2-3 blocks, etc. You can drop a few hundred to a few thousand on tooling easy. A DRO, if not included with your mill, is a great thing to have and that may cost as much as the mill in some cases (but can be done more cheaply).
Still with me? You can see that if you're just doing this one lower, that AR45 is going to be a very, very expensive carbine
But if you're getting into this as a hobby and the AR45 is just the first of many metalworking projects to come, it may be worth it to you.
Hope this helps.