If you have routes with demand of 5000 that are unserved, I think you'd be better off using the DC9 profits to fund a widebody fleet as with the departure frequency restrictions, its probably impossible to offer that many seats with a Diesel. I don't see the F100's as a significant upgrade over the DC9 anyway - the economics of both aren't great even when fuel is virtually free.
Given the natural demand increases over the course of the scenario, I think you'd want to consider a larger plane than the F100 as your replacement fleet type as I would expect to see at least double the demand you're currently seeing in 10ish years. I would suggest replacing them with MadDogs, 734s or A320s as they become available. If you really want to replace them before they're available, go with 722Advs.
For widebody selection, anything should do, and I'd suggest HD seating for routes under 2000nm. The L1011 tends to be the least popular widebody in this scenario and is the most available in the early game, even though it has comperable economics to to the DC10. A300's would also work well for you though (and they're not usually popular until the -600 is released).