I usually buy Garrison and Auto-Turrets first turn, and keep both available throughout the whole campaign. Those two bonuses are cheapest, and against AI especially, probably the best. The rest, except Hero, are practically worthless. Bacta Tanks' regen speed is too damn slow, Armor only adds 5% health(enough to stop a pistol shot at very best), Supplies is only good for sniper/assaults(the rocket guys), Sabotage can be very useful on certain maps or in space(if the AI uses Marines, who can't repair their fighters). Hero itself can be useless depending on the hero you actually get(map/faction dependant), in my opinion, Jango and Boba Fett are nearly worthless, as are Leia, Han, and Chewie. Aayla Secura is both deadly and sexy, though.
For units, I'd reccommend engineers as well. Their shotgun can be even better than a blaster at close- and mid-range, they can repair friendly vehicles as well as health/ammo driods, hijack enemy vehicles, plant bombs on vehicles and destroyable things, and self-replenish health and ammo. I'm not sure, but I think CIS Engineer Droids can repair their comrades as well, I know for a fact that the Jawas will do it. If you're playing single-player, avoid Marines. Even if you do go for boarding action, the AI tends to either dick around on space maps, often not shooting at each other. At least once, I could have sworn I've seen one of the guys spawned from a gunship actually fly off in it, leaving me without a spawn point in the enemy ship. Not that it really concerns me, I usually land a fighter in their ship, bomb their shield generator, then bomb the auto-turret computer, the engines, and life support, then bomb a few of their fighters, then steal one and fly off.