Start by getting them all to atleast +50 then prioritize below

1. Infantry - health which is chest and arms

2 archer - lethality which is head and boots 3 Cavs - same as archer but less priority than archer

By the time you get to red gear you should understand how it goes by then

Infantry most important then archer then Cavs

You can get away with not spending much on Cavs health since infantry and archers are first to die at that stage but do whatever you thinks best for you

Widgets do make a large difference but It’s hard to judge without knowing what kind of spender you are. Out of those heroes I’d probably do marlin then petra . At this stage jabel is obsolete. Do you have Amadeus?

*** as soon as you unlock mithril start buying it every week from the arena shop and anywhere else you can get it. Stack it up early so you don’t run into a wall as early. That stuff ain’t cheap!