It is difficult to build a all in one appliance - A dedicated rescue vehicle is the way to go in my opinion.
If a Fire Appliance with RCR and 4WD capability is required trade offs will have to happen. It also takes a lot of money.
Another dynamic is the range of RCR gear that can be used. Also the role (heavy rescue
If somebody could ever define a minimum and have it agree to across the board (lighting, generators, RCR gear, pump size, water capacity, seating capacity, 4WD or 2WD, foam - type and amount, crew numbers for tasks, size of vehicle - GVM and dimensions.)
I may be also a little bold and say egos and oppions drive a lot of this type of debate not reality and practicability. (That being said, I do not have an answer for the problem)
Good luck!