I was into this a lil, and i just sold my old good Tippmann A-5, with little modifications. I still have nitro-tank (I'm selling it BTW).
However i won't recommend you to get Tippmann 98 for only one reason, after 2-3 times of use, you will be dreaming to get something newer, more accurate, with some electronic feeder, electronic trigger, nitro lines etc. Everything is expensive. So I'd get something for more than $600 with everything build-in. plus another 300 for gears and uniform. All prices are average, as the saying goes "And the Sky is the limit".
When it comes to realistic part, did you think about air soft guns?