Never played a drone-centric engineer. I might have to try that. How would you recommend building them?
It depends on the play style and matching the drones evolutions to that playstyle.
The drones will alternate between all the attacks available to them when targeting an enemy at a certain range. For example the Sentry Turret will alternate between it's Shock and Flamethrower evolutions for enemies within the turret's melee range (assuming both are chosen) the Combat Drone will alternate between it's rank 5 Shock Evolution (which is completely separate from it's main attack) and it's main attack.
Shepard's Power Damage bonuses will increase the damage of all the drones attacks and abilities. However, the rank 6 Tech Vulnerability evolution of Sabotage will not effect the drones' damage.
The Combat Drone's rank 6 Rocket evolution will change the drone's AI. The rockets need a minimum range of 4m to fire and a Combat Drone armed with rockets will back up to fire at enemies, sometimes the enemies will chase them (which is pretty funny).
For the Sentry Turret I always choose:
Rank 4: Shields and Damage
Rank 5: Armor Piercing
Rank 6: Flamethrower
The best part about the sentry turret is its Flamethrower, it does good damage, panics enemies, and can prime Fire combos. To best utilize this I constantly reposition the Turret to enemy spawn points or a cluster of them. Because of this I choose the Shields and Damage evolution to make it bulkier, also with the Shock evolution the Turret will alternate between shocking and burning enemies which I find to be inferior to the turret simply keeping open the door to hell and setting everything within range ablaze. I've never seen the Cryo Ammo evolution freeze anything while the armor piercing evolution will increase damage and penetrate Guardian shields.
The Combat Drone is a bit trickier. I personally don't like my drone playing the martyr (or Voltorb) but a self destruct build should probably look like: Detonate/Shock/Chain (rockets will cause the drone to run away and not explode next to enemies), this will insure that the drone charges enemies and explodes quickly. I personally always choose Shields and Damage for it's rank 4 and 5 evolution. The rank 6 evolution is where the drone gets specialized. If you want it to deal damage choose Rockets, if you want it to serve as a pseudo tank choose Chain.
I always play my Drone Engineer as a shotgun engineer. Utilizing the drones to distract enemies so Shepard can close in and Cryo Blast to freeze them. For a bonus power you can choose Defense Drone if you like to keep close and keep up the drone theme, Dominate for more minions, or Defense Matrix.
And the final step is to name your drones. I have Dribbles (Combat Drone) and Shoots (Sentry Turret).