Ion Ups...
All the recomendations for ups are good, but you need to think about what you want. If your on a limited budget I would recomend performance and practicality upgrades only, and would do them in this order.
Q-lock feedneck - Keep the damn hopper in!
CP Oops - Less wear on tank thread. Makes it easier to screw tank in and out.
Freak barrel system - Better bore to paint match = better consistent accuracy (if your on budget just get largest bore only)
SP 360 QEV - Makes the gun work more efficiently. You use less air!
Lucky Stage 1, 2, and 3 - Stage 1 = bolt, Stage 2 = Firing Can, Stage 3 = Donut. Will make your gun cycle smoother and again use less air.
(QEV and lucky stages require adjustment of the dwell which is easy peasy)
Triggers - Before you change the stock trigger (which aint bad if you set it up properly), have a go on someones Ion who has an aftermarket trigger to see if you would benefit from the change.
Board - I personnally wouldnt recomend a new circuit board unless you got a money tree in your back garden. The gains aren't much when compared to the £ spent. (No flaming just my opinion guys
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Cosmetically, anything you flipping want! New body kit, grips, whatever...
My Ion = Black ion with: Q-lock feedneck, CP Oops, 14" Freak barrel with .692 bore, Lucky Stage 2, SP 360 QEV. Next Investment will be Lucky Stage 1 and 3.