[Drakker]

PDI V-Trigger with vacuum cylinder/piston kit. Install an airbrake like the ones Thundercactus makes.

Keep the stock hop-up chamber, but use an upgraded hop-up arm like the dangerwerx, maple leaf, airsoft pro, etc. Any of them will work. The Maple Leaf Autobot rubbers are amazing with the maple leaf arm, but you could also use an R-Hop.

Lastly, the barrel. If you use a maple leaf rubber, you need to unbridge the barrel for it to fit, if you R-Hop, you keeping the bridged barrel could help. The stock barrel on the G-Spec is actually pretty good, so you can opt to keep it. If you change the inner barrel, then a Laylax/Prometheus 6.03 or a PDI (6.04 or 6.05) would be your best choices. It's not worth it to put any other brand in because they will not perform that much better than the original barrel in a G-Spec (note: the pro-sniper has an aluminum barrel, and it must be changed, hence why all the guides around will recommend changing the inner barrel). You might want to use barrel spacers too, old school thinking said that the more spacers you used the better, but lately the thinking is more that they don't do much and that you don't need to worry about barrel vibration in a slow shooting bolt action rifle anyway, so two spacers at either ends of the inner barrel are probably more than good enough.

That's it, this should fit in 500$.