Grizzly has good stuff, but i haven't looked at a catalog in like 5 years
Get yourself a Grizzly machinery catalog. lathes/mills can run from a grand to tens of thousands.... south bend is about as crazy as you can go.
on the way lighter end look around for a Smithy lathe and mill, its a dual set up (belt driven) . You can usually find them cheap. Those are good for small stuff like a ar-15 bbl or even threading and chambering sporter barrels.
i may be over exaggerating BUT be warned
If you have never worked with a lathe consult with someone personally that has worked with them, another thing is do NOT wear over-sized clothes or you will regret it. When the 3 or 4 jaw chuck grabs your shirt, it isnt fun at all. Iv seen people get killed screwing around with lathes... even small belt driven ones can hurt you.. Please don't let that affect your future hobby, as free hand machinists are fading fast.
The guys above are pretty spot on, Unless its for a friend or family. who really cares if you milled out a lower. As long as it has the serial# on it and it isnt hot, your fine. Remember to check state laws. some states are as****es about stuff like this. Make sure you do not plan on doing it to make a lot of money... or its wise to have the above.
Good luck and be safe:
Jim