Once you start farming mods you will hit the credit crunch.
It costs roughly half a million credits to upgrade a 5 dot mod to level 15. If you only do mods for your arena team you need 30 mods.
Then the cost of each level increases as your level increases.
In your last post you appear to be mixing up dark side and empire. There are numerous dark side factions. Kylo ren is first order not empire, boba fett is a bounty hunter not empire.
As to what empire characters you need well tarkin and tfp are needed for ships so I would work on them anyway then add a tank (storm trooper or Royal guard).
That said Vader Palpatine and Thrawn are key to any top tier empire team. So there's 6 to work on.
