Hi, manufacturers specs 16,000 pages @ 5% coverage. Realistically if your envelopes just a logo with some address lines, you should get a 10K out of the set, with higher coverage you will obviously get less. By-pass tray envelope feeding is fine as long you OK with feeding 0.5"-0.75" stack at the time. Basically having 2 printers, you can have 1 person feed them both and be somewhat efficient.
Simple math is like that : to run 100,000 prints you will need 1 fuser $300, 1 belt $300, 3 sets of drums $1800, 10 sets of toners $7000 TTl $9400 which makes it cost per click just a little shy of 10 cents. This counted everything OEM, if you use aftermarket toners or find an awesome deal on OEM ones, this could be cut somewhat.
Also keep in mind that once you start running envelopes, they will mark the drum surfaces corresponding to envelope's oughter edges so mixing of envelopes and pretty much anything else may not look pretty at least not on the same set of drums.