ok well as your going for an ion there is 2 main upgrades i would recomend
1:Q-lock feedneck, depending on what hopper you gun will will need otn off thease
2: Detents, this is what hold the ball in the breech ready to be fired(stops it rolling out the barrel) the stock ones on ion ware down very quickly(i have not used a ion from latest batch they may have made them better)
and the next cheapest upgrade is a QEV, i quite remember what this dose officially but it alows the bolt to cycle quicker so you can lower Dwell on chip-board and make gun more efficient