These are the specs that are going to be most directly impacting configuration:
Payload 80 passengers
Crew 2 pilots and 2 cabin attendants
Range 800 nautical miles.
Cruise altitude 35,000ft.
Takeoff field length 4,500ft at sea level
Cruise speed Mach number = 0.80
Powerplants 2 turbofans.
Whoever wrote the spec has been a bit of a 'tard as they've mixed two market segments. The cruise speed, altitude and engine requirements represent a full fat turbofan airliner (737 etc). Your runway length, crew, pax and range are more typical of a turbofan (ATR72 etc). As you may well know, over such short haul routes the cruise alt and speed are of less importance so they'd tend to cruise in the 15-20k ft range rather than climbing only to then descend. You really want 2) in the cruise and 1) for takeoff and landing.
If it were me i'd be looking to do something CRJ-esque. The CRJ as it stands wouldn't meet your runway requirements but would meet cruise, so here's what i'd do:
- Take a CRJ clone
- Pump it full of composites and avionics. (A350 panel construction rather than 787 'all in one' composites for the fuz)
- Fit a larger wing covered in high lift devices to meet the take off requirement and include space for larger fuel tanks.
- Call this the base "100" model.
- Offer expansion in the form of "200" & "300" stretch models offering more pax (say 100, 120) while still using the same wing (i.e. longer take off runs for them).
- Use the promise of future expansion to write off part of the aircrafts development costs to later models, giving a lower purchase cost for the 100.
- Undercut the price of the other teams while meeting the spec and presenting a clear future development strategy.