I know you've had alot of experience with air rigs and such since you played P-Ball quite a few years, but I would really recomend that you go for a spring powered BASR before you get into gas. A gas rifle requires alot of time and dedication, not to mention money, for it to begin to get amazing results that you may have seen from others on this and other forums.
I started with a JG Bar-10 which I soon upgraded using ASGI parts, it turned out to be a solid platform for some awesome games, getting about 250ft of range I would have alot of fun. It ran me around the $250 mark including the gun.
So personally I would recomend starting with a spring powered rifle first.
Oh, and welcome to the sport!
Yes! Welcome to the dark side! (or maybe just the grown-up side of mil-sim)
I would have to agree with this though as well. There are a lot of differences with airsoft that you don't have to worry about in paintball e.g. the hopup (along with everything else that goes along with it). You will find quickly that there are many many quirks that you'll have to learn about and figure out with your gun to make it accurate.
HOWEVER! With that said, and bearing in mind that you want to go gas. Why not go with a gun like a spring VSR or an APS2 that won't cost you that much up front, and down the road you can rig an after market gas cylinder in it. That way you'll also save a ton of money on mags running a remote rig with gas through the bolt instead of through the mags. Just my 2c.