So here is my "Test Build" As you play you will figure out what you want more.
Basically Infantry for Turret damage, increased fire rate and less recoil on kills, turret cooldown and missiles from turret.
Support so you can regen ammo by turret, increased number of turret shots, and shield regen on kills.
Medic so being by turret heals you, and increased magazine capacity.
I would say you want the ammo regen and healing by turret first. But it really depends on what kind of damage you are taking. If you are hard up for skill points or want to spread it around more I would put only one point in the "kill" related skills until you are comfortable. It is 5 in each row so you will have to use your digression. I did not play Roland as much as I should have, but I think this is a good template as you go forward.
Hope I didn't take to long.
Good luck.