I tried using a brutix on lvl3 missions but lost it pretty quickly
. Though my skills were pretty poor back then. If you're armour tanking its probably a good idea to fit a cap power relay in one of the lows in addition to the cap recharger, you'll need all the cap you can get to run the armour rep for as long as possible. Using tech2 or active hardeners will also be a great help but I'm not sure if you'll have the grid/cap to run them.
My advice would be to train up for an enyo; its fast, hard to hit, has great resists, lots of firepower and tanks brilliantly
Done every lvl3 mission in it, spy stash was the only one that gave me too much trouble. Though some of the longer missions like the extravaganzas do take a while. Its cheaper than a brutix too, but you can't insure it properly.