Milling my first 80%. I have access to a mill and have a jig.  I need to get a bit and a way to prevent over milling for dummies.  There is no stop on the mill.   Under 50$ would be appreciated.

if you have a jig, why not just get the one recommended by the manufacturer?

Exactly, get their end mill to fit whichever of the supported routers you acquire.

Sigh.  I have a mill,  and am looking for a mill bit recommendation.   My jig is 3d printed.   I am teaching myself to do this without paying for jig kits. What mill bit and collet stop are people using.

Good luck.

I used a 3/8" center cut end mill, and a 5/16" end mill for the trigger slot. Cheapies should work fine in the aluminum.

Save yourself some time, and hog as much out as you can with a drill bit first.

If you have a mill, and want to save money, make your own drill stop, or mark your scale or dial.

This aught to be good...

Do you intend to work strictly from just the jig, or do you have drawings to get/check dimensions?  If you don't have already, download the Ray-Vin manual on machining a lower.  It's written for using a mill.

As for tooling, at a minimum, you should have:

5/32" Drill - Trigger/Hammer Pins
3/8"   Drill - Selector Pin
1/2"   Drill - To remove bulk material from the Fire Control Pocket (the 3/8" will be used for this too)
1/4"   Drill - To start the trigger slot
7/16" Extended End Mill - You have to machine 1-1/4" into the FCG pocket, not to mention your jig thickness (a 3/8" will work, but it changes a few things)
5/16" Extended End Mill - For the Trigger Slot

Now, some comments.  If you have a mill (and a milling vise), you don't need a jig.  You can do everything to finish an 80% lower without one.  If it doesn't have a quill stop, but does have a table that raises and lowers, you can use the "UP" position of the quill as a stop.  You raise and lower the table to cut, and count your handle rotations.