If it were me, my first approach would be to ensure that you get room tone for noise reduction; funny how on run-n-gun corporate shoots, I've sometimes seen that totally get overlooked. The success of this will depend on if the subjects are moving; I'd get room tone for each shooting angle/location to be ultra-safe. Since that's all focused around fixing in post, though, it's hard to know how clean that'll make your dialogue. iZotope RX is stellar for such things.
This would want to be to be paired with a carefully tech scout of the location with a facilities manager and see if you can put sound blankets around to deaden the sound in specific places. Work with the producer and DP for shooting angles that don't force the lavs to be exposed directly to the fan noise. Grabbing room noise for noise reduction during that tech shoot, using the lavs you want to use, wouldn't be a bad idea if you can spare the time.
If these top two techniques work, then it's pretty much whatever lavs sound good on the respective voices. MAny love the COS-11 and DPA 4060's, but just as many love cheaper models like the Trams and Countrymans.
A final alternative might be to get cardioid-pattern lavs. These tend not to be too popular, but they do have their place in really challenging environments. You'd HAVE to rent some and do some tests to see if that'd really help your situation, though, before committing to a purchase.
Just a handful of initial ideas...