Ok — these are my thoughts
Fast Magni will do more for you in most places that slow Isarnia — and you’ll be ahead of the curve if you pull a costume. Can use Emblemed Grimm or Wilbur in the meantime in many places.
Marjana is one of the only red snipers, and her costume will be quite good.
Personally, I’ve never really considered Mushashi — he’s competent but lack lackluster; I’d rather keep the darts… but he can be a solid hero if you’re limited on options. Very high tile damage.
Alberich is only available in Legends — not worth chasing but you may get lucky. I love mine; but don’t use him as much without emblems. I got him when it was a lot easier.
Heimdal is great - he’s my tank, but I wouldn’t chase him. You may get Frigg, other stuff or the meta may shift. Pull at the end and get what you get.
If I was chasing anything — it would be C Leo and C Marjana, with lots of good little consolation prizes sprinkled in that keep s1 heros current.
Aeron is polarizing —his shield is fantastic enough some folks have multiple leveled… Others leave him on time out at 1/1. The heal, cleanse, class is a plus. Mine is at 3/70 — as there’s always been other options.
I’m a fan of play what you have, and you have a lot — others can get into it. I’m not a fan of leveling for specific uses when your building… get a core together that lets you finish everything that gives mats, emblems, expand for Titans then use that to build a war defense online with your alliance.
Don’t forget troops and infrastructure — if I had it to do over I’d skip a lot and barrel through to level 29+ on mana troops
Hel is cool — but Proteus is attainable. If you can find him, keep all copies — save the tabards. It’s functionally a mini me
Boldtusk, Kiril and Caedmon will serve you well for a long time - and their costumes are great. Get the 4* up — they’d are the backbone.
Gormek will pair with Falcon and Marjana very well, but will be replaced by Wilbur of you get him. He’s worth the knives —- seems like a lot now, but you’ll be tripping over 3* mats before you know it