It's funny, I never thought much about the gaming side until I went to a class given by a local guy who does it. Turns out you guys can teach us a lot about getting a decent paint job!
As for your questions, your best bet is either 1/48 or 1/72. Those are both pretty standard for aircraft/helicopters, and I know you can find at least some (maybe not a lot) of armor/vehicles in those scales. Standard scale for armor is 1/35, but I don't think any aircraft are made in that scale.
And remember, the smaller the scale, the more detail you can put on it (ie: 1/48 is bigger than 1/72.)
For brands, Tamiya is usually considered the best all around. Excellent detail, easy to build, lots of subjects. Their prices are just slightly higher than mid-range. Trumpeter has very excellent details, but tends to be pricey. Academy and Revell are middle of the road priced, and some of their kits are very good, others are sort of bleh. Best idea is if you have a particular subject you want (such as an Abrams tank) just ask around for people's opinions on different kits.
But welcome to the forums! With your experiences on the other side of the hobby, hopefully you can teach us a few things!
-Budd