Early get first 3 gun upgrades, then 2 spirit, then 2 green to help win your lane. upgrade the rest according to how you need to play. get healing rite if you desperately need health constantly or a teammate do. I keep quicksilver for pretty much the whole game.
I500
I500
I500
I500
I500
I500
I500
I500
I500
I500
I500
I500
II1250
I500
Mid Game I have put these in the order i usually buy them or their importance to me. healbane and decay to help against lifesteal if needed. If other team is going mostly gun or spirit you can buy armor to help with survivability
II1250
III3500
II1250
II1750
III3500
II1250
III3000
II1250
II1250
III3500
III3500
II1250
II1250
II1250
II1250
II1250
II1250
II1250
Late game These are also KIND OF in order once you play enough you can upgrade how you please. In End Game start focusing on cooldowns, and gun. make sure to have lifesteal and siphon for lifesteal and boundless to increase spirit.