First, you’ll need the basic upgrades, common to most of the loadouts. This is the suggested order:
Barrage level 1 (100 certs) OR Fire Suppression level 1 (30 certs).
Barrage improves firepower situationally, but fire suppression is more forgiving.
Trial top gun of your choice.
This will allow you to use
Racer 1 (100 certs)
Stealth 1 (30 certs)
Gatekeeper (550 certs)
P2-120 AP (875 certs)
Reload upgrade 1 for AP/top gun (150 certs each)
Gun magazine sizes 1-4 for AP/top gun (1/10/30/50 certs per level)
Gun optics of choice for AP/top gun (1-500 certs each)
From here invest evenly in upgrades as certs become available.