Not sure what ship you're currently in but if you're shooting for bb6, I would do atleast 4 engineers, and atleast 3 reps. I leveled a restorer as well to reach the SD cap faster, SD cap for me came around level 90. I would recommend it as it helps with the rushing ships to pull range with some survivability. Don't worry about having to level an engy or rep to replace him in the end, as the road to bb6 is long. This brings me to your next point about aa being useful; infact it is very useful. MN has very good aa range and a large selection of aa guns, most if not all of their bbs have good aa platforms. I would recommend it as it will really help in leveling.