What great heroes you got - welcome to the game
If I were you I would focus on fast Heroes first. Then Average, then Slow, depending on their Special Skill. Heroes that mana control, have defense down, or attack and defense up, cleansers and dispellers are all great tools to have.
I´d prioritize the heroes that are somewhat sturdy compared to fragile ones (like Muggy. As great as he can be, he needs emblems and what not to not die fast)
That means, for me, I would do these first:
3 heroes
Pixie (mana control), Vodnik (speed), Bauchan (mana control), Isshtak+Costume (defense down), Brienne+Costume (defense down, attack up), Belith (dispeller, healer), Rekhetre (she´s new and fancy and I dont know her but I’m sure she´s great- category!), Helo (healer and cleanser).
4 heroes
Sun Shangxiang (she´s new and fancy and I dont know her but I’m sure she´s great- category!), Kelile + Costume (speed), Cheshire Cat (speed, elemental defense down), Guardian Jackal (elemental defense down), Wang Yuanji (she´s new and fancy and I dont know her but I’m sure she´s great- category!), Chao (speed), Scarlet + Costume (speed).
I wouldn´t level 5 heroes yet
Once I have levelled those, I would look to my slower heroes, and see which ones would fit well into the category of the weekly tournaments. Preferably those with costume, like Gunnar, Jahangir, Hu Tao.
I might, just might, be a tad worried of your match making in tournaments if you make a few selected heroes very strong with lots of emblems instead of spreading them out a bit. I have found myself very overwhelmed in the past, doing that. Because the system takes your strongest heroes, and then match you versus the opponent with a defense that have the same power as the strongest ones of your collection, I mean.
But that´s just my approach though, and I´m no expert at all, bear that in mind
I hope you will enjoy the game, have fun