For varminting, plinking and some target work. A 4-14x, 4.5-18x is the general compromise. At least that's what I've gone with on my rifles.
Here's a break down of the magnification levels that I chose:
1) Bench guns. I plan on getting a 36x
2) High end varmint/target rifle 6-20x Leupold EFR
3) Semi auto Varmint/target/plinking Falcon 4-14x 44mm Note: I have a 4.5-18x Falcon on my Thompson R55, I upped the magnification a bit due to the known accuracy of these rifles
4) Hunting/plinking, 2-7, 3-9x red dot Or iron sights.
Yes, get a scope with AO. Especially if you plan on doing any target shooting with it. I've used a non AO scope and even with a consistent sight picture, it was frustrating.
There are a few scopes out there that have a parallax adjustment down to 10 yards. 20 yards is common for many indoor ranges. Bushnell Elite, Falcon Menace, Leupold EFR, have this feature. I think some of the Sightrons II might as well but can't confirm at this time. Zeiss conquest lower magnifications only go to 30 yards, while the 6-20x are at 50 yards minimum. So be carefull about this.
Other things to consider, are the crosshairs. I'm a fan of fine duplex, NP-R1 (Night force), ML-16 (falcon) reticles. Make sure to check the different reticles available.