Rust Lung and Harbor are great Engineer Levels IMO. I'm not the best Engineer and I don't consider it a fun class for me to personally play but a Scout and an Engineer are almost always the 2 crucial classes needed for an optimal horde team game. Engineers are also the easiest and quickest class to get all your cards to a full level 6!
I do love me some barriers though, so here's my build with my Engineer class. Common green (Build Cost, Fortification Health, Barrier Cost, Barrier Health) and rare blue (Sentry Cost). Most people like repair boost because it speeds things up but I find it more cost effective to skip it. Turret skills used to be "OP" but they've been nerfed so you can ignore those. A lot of people swear by Weapons Lockers skills but I'm not a fan of them. Carry speed can be a good skill for speed runs though.
As always if you have questions hit me up.