Upgrading from ultimate starter kit

Hello! Me and my gf bought the ultimate starter kit because I wanted the ultramarines and she wanted tyranids. However, I'm at a lose on where to begin for upgrading my army. There's so many choices. I know I want some kind of tanks or mechs for sure but not sure what is best.

(I will paint all of these before upgrading but wanna know about my future options lol, it's all very exciting)