[lordnordeth]

For breakdown style rifles, the KJW M700 and the Tanaka M700 are the only ones I know of (the KJ has its issues but it ok, any tanaka is apparently worth it's weight in gold), both are Gas BAR's, and yes, gas will be affected by environment (although if you convert to HPA the effect will be lessened).
As for "springer" BAR's, there are as many opinions as there are people around here, each has their likes and dislikes, but the one universal rule...kiss your free money, and time goodbye. Buy a decent rifle that appeals to you, and then make friends with a gun-doc who will teach you how to do the work and adjustments on your rifle as you proceed (that way you are learning to fix your own kit, and arent screwed if you have a failure in the field).
Expect to invest a lot of time, money, and effort into a BAR, or don't even start. If you half ass it, you'll be disappointed, and end up hating it, do it right, and you'll love the role, the rifle, and the dirty looks you get from people when they realize you just shot them out with a springer (incidentally, I have about $700 and 40+ hours tied up in my A&K SVD, and that is just to make her field ready)