I'd wait on Kerrigan.
I love Sonya, even though most consider her underpowered. She's less meaty than the tankier warriors, but she can still take a beating, and has ways to keep her health up. Her non-reliance on mana also gives her great sustain without having to Hearth back. With Wrath of the Berserker, she get a 40% dmg boost which she can maintain almost indefinitely if played right, so she packs quite a punch too. With the right talents, she can solo bosses too. She's not for everyone, so i'd suggest trying her out during her free week first
Rhegar is a great support. Good healing, and his wolf form is really great (good for chasing and escaping without getting knocked out of your mount)
Tassadar is an assassin-support hybrid, with his AoE dmg spell and his shield. You just have to cast the shield before the dmg lands. He doesn't replace a healer, though. I quite like him
Hearth often and be aggressive with your mana since you can just Z back. Polymorph is great for shutting down melee assassins that try to engage (illidan butcher etc). The tier1 talent to reduce Z cooldown is nice, as is the shield granting Z. Double pixie dust (esp with 50% speed boost or half ability dmg) is cool
I think I'll pick up Sonya and Tass then, both 4k so I should be able to earn enough gold for both within the next couple days... and then my gold earning dries up cause I'll hit Player Level 40 haha