What Compressed air tank should i get for my new Proto SLG paintball gun?

If you've got the cash, I recommend yu get a Crossfire 68/4500 HPA tank. They have one of the fastest recharge rates and are pretty good in quality. However, even with a HPA tank that has one of the lowest recharge rates you'll still be fine, as your marker would have to be firing well above 30bps to be affected. As for upgrades, there are many things you can do. You could buy a different barrel, trigger, bolt, board, eyes, grips, feed neck, detents, regulator, ASA, drop forward, and so on. The list of things is quite long, however you don't need to replace anything unless there is an issue with the stock part or lack of. Here are some examples, the grips are uncomfortable, you lose your hopper a lot in mid game, the board doesn't have enough firing modes/low bps cap, or you want a drop forward to help balance the marker. Whatever you replace and why you do so is completely up to you.

VVV i would just get the clamping feedneck

Any hpa (compressed air) tank is worth the money. Get whichever you can afford. There's steelies, which are steel hpa tanks. They typically hold less than their carbon fiber counterparts & are a little heavier but they're way cheaper, like half the price. Carbon fiber tanks are lighter & come in a wide variety of sizes. But they're $150-250 each. As for upgrades, you can get triggers, feednecks, barrels, backcaps, bolts, asas, regs, boards... really though, you don't NEED upgrades for the SLG & honestly i'd recommend keeping it stock and then getting a better gun in the future.