If you already have the scope and all of the "other" goodies, and you are set on a gas rifle..... why not take a look over at the KJW M700..
This rifle is able to take a VSR inner barrel right off the bat for you, and I am assuming that if you have been paint balling, that you already have an external air rig for your rifle? If so you can tap your mags and be fairly well set.
The VSR barrels are VERY easy to find in a wide range of sizes and length. And they can be found for fairly cheap as well. So this is a good thing for you.
Short mags that you can use for the rifle.
Here are the long mags if you would like as well.
Now is there any reason why you are wanting a gas rifle for your first?
If you are looking to spend around $250 for the entire set up, This is what I am going to suggest, as a secondary option for you.
This one is darn near an exact clone of the Tokyo Marui VSR rifle. So any of those parts are going to fit into this rifle. And you can find the parts at just about any airsoft retailer that is worth there snuff
I know this one is a little bit more expensive, but it is worth it as well. This rifle is compatible with many APS-2 parts, and much like the VSR parts, these parts are very easy to come by as well.
I have owned maybe a half dozen of each rifle. And all of them I made them to do different things. I had some there were long, short, shot extremely fast and some that shot extremely slow. And they were all VERY easy to work on and find parts to. That and there is a wealth of knowledge not only on this site, but many around the web as well.
Let us know a little more about what you are planning on doing and we may be able to help you out more and more.