I've been putting stats in everything but dexterity. The main issue is "Focus" and how much it applies to ability damage, or how much it actually boosts your overall DPS. There's no calculations for it, and tracking down magic damage is pretty difficult.
"They have got their **** in the bag!" R.L. Ermey's thoughts on PC Gamer.
I'm assuming you're going to use cannons. In that case, I highly recommend you invest in the first passive in the Blitz tree (Heavy Lifting). It stacks nicely with the 35% chance to stun that cannons get by default.
Blast cannon will probably be your main skill for a while. Get Heal-Bot to the first tier at the very least. The mana regen is well worth it. Spidermines are ****in' adorable but they haven't really been that useful to me. Fusillade is, well... You basically empty out your mana pool for a huge damage spike. It isn't very consistent (or efficient) unless you're willing to spam pots. It's nice when you absolutely need it, but I'm kind of reluctant on recommending it.
Gun bot is nice to have, but I rarely used him outside of boss fights. Sledge-bot is more of a distraction more than anything once you get him (on elite, anyway) but he's nice to have.
fusillade is terrible, it will miss about 75% of the time, your main cannon damage will come from cannon blast, and you dont need to put any more than just the base point into it. your weapon is mostly for constant AoE stuns and the damage is secondary.
i dont think any stats effect minion damage, only YOUR damage, so you need gear that gives + minion damage. that not only effects your bots and your pet, but also your summon spells AND your pet's summon spells. so being fully decked out with + minion damage gear will be your damage output.
my engineer is 79 on elite and i my little army of bots and skeletons clear out mobs quickly, the only danger is bosses because they completely ignore the minions and go directly for you.