Amanda, You've done a great job of fact-finding but there are a couple of things I would also want to know before deciding on a recommendation.
1.) How many pages per set is typical? If we are talking less than 10 then I would still consider a duplicator with off-line collating. If they are 11x17, then a bookletmaker can be attached. We double-click 11x17 done on a copier but not when done on a duplicator. A copier will also slow down to half speed for 11x17 but a duplicator does not. Two 2105's running in combine mode are still slower than a single duplicator. The duplicator will need more attention however...in less than 10 minutes the paper tray is empty.
2.) Is there any applications that are long-run but non-manuscript like flyers or NCR forms. At their volume, the savings generated from 20% being shifted from the copiers to a duplicator will probably generate enough savings to pay the lease of the duplicator. In other words, the copiers and duplicator combined are less expensive than the copiers can be by themselves when lease, service and supplies are all taken into consideration.
If a duplicator can provide a value add without adding to the cost, then you have just eliminated much of your competition...especially Xerox and Canon.