Let me tell you something:
First, you have to think seriously do you want to snipe in airsoft. Its much harder than it looks like. Really. It much much much much easier to play with an AirsoftElectricGun than bolt-action sniper.
Second, you have to choose do you want a VSR-style sniper or L96 style.
If you want VSR-style, buy Jing Gong BAR-10 or Tokyo Marui VSR-10. You cant go wrong with the TM because, hey, ITS TM
And all the upgrade parts are designed to work in it. Better tolerances also. Jg bar is easy to DoItYourself to 500fps. So that will be cheaper.
If you want L96, I cant help you with that because I dont own one. Mostly is good to pick a brand, which the parts are designed for. And that is usually Tokyo Marui.
Uhh, realistic mag placement? Well...
No idea which is considered as realistic
All the snipers?
About the upgrades : Most important is to have AS ACCURATE AS POSSIBLE sniper and after that comes range. First upgrade should be bucking and inner barrel. Dont buy the cheapest inner barrel what can you find. Tolerances are important here. Prometheus, PDI, Laylax, DB customs and edgi makes good ones. For the bucking, i think most of the buckings will work. Your bucking should be hard I guess when over 500fps.