I focus mainly on defenses at first too, but getting MRS and some mk2 ships is a high priority too on my list.
It's good to take a look at what ships the AI seems to have most, check their hull type, and get some higher mark turrets to counter those. For instance, in my last game, nearing the end the AI's used almost exclusively Acid Sprayers, so I had all flak turrets and fighters unlocked, since they seem to do well against those. Fortresses seem to give a nice boost to early defenses too, but they can be easily overwhelmed by an oversized wave, and the low cap makes sure you can't have 'em on every planet, so I mostly use them to back up main choke points, if I decide to unlock them.
I tend to always take Mk2 bombers at the start. Makes clearing the neighboring planets somewhat easier and faster. In the end, I get knowledge from those planets, so I don't tend to worry too much with the very initial unlocks. I can plan out further unlocks from there on.