My first suggestion will be the Provari,
ProVari Variable Voltage Ecig: Best Variable Voltage E Cig Available! get the full size, not the mini and the extension cap for the 18650 batteries. Check the clearance section, you may be able to get a blem for a little less. But it's going to cost you! But there isn't anything much more durable for your needs on the market.
Second suggestion is the SS ZMax, make sure you get the SS!! This is about the going rate on these, but sometimes can be found cheaper,
Smoktech ZMAX Stainless Steel Variable Voltage/Power APV Not as durable as the Provari, but seems to have the fewest problems of the Chinese clones.
Then you will need some batteries, for you I will strongly suggest you use AW batteries in either one, just because of your use! It's not worth saving a buck for something that might not work on a rig when you need it! Spend the extra couple of dollars for the quality! I'll also suggest you get four batteries, I've heard of you guys working 18 plus hour shifts, so two on the charger, two with you should get you done!
Good prices and a great guy to deal with,
AW IMR 18650 2000 mAh I won't get into all the details, but I wouldn't skimp too much on the charger either, the Nitcore is good for the prices, the Xtar is new, but about the quality of the Pilla, but the Pilla is still king of the hill!
Battery chargers at RTD Vapor
But of the mods, those would really be the only two I'd recommend in good faith right now.