Greetings Petey1987, and welcome aboard.
Yes, of course you can use a 3-9x40 on an AR; however, they are not all created equally. The grey beards will remember the initial Colt AR scopes were very compact fixed power units, typically accompanied by a stock riser. The flat-top uppers and adjustable stocks have changed things. Years ago, when I scoped my carry handle AR, I selected a variable Burris compact (IMHO, the Colt scopes were outrageously priced). There are several Burris scopes available in the $200 neighborhood.
For night work, my old eyes are happier with some internal illumination, YMMV. You mentioned 150-yard shooting. In reality how far will you see at night with lights? Perhaps, you'd be happy with a simple illuminated dot 1x unit. Of course, there's nothing that says you can't have two units: magnified optical during the day and a simple dot at night. Both can be had for under $200.
