+1 search suburban tahoe GMT400 or GMT800 here, and you'll see a decent amount and levels of builds with some documentation. I would start at getting your rig up to a high level of maintenance, researching the weak points and fix them permanently or make some space for spares (not sure what that is on GMT400), and then see your needs and then associated weight and then determine tire sizes, lifts, winches, spares, leveling/ weight management, upgrades etc.
I would also recommend the "real" 12v fridge with dual battery and solar. I moved to one, and it is amazing not having to worry about ice (and also you can carry more because the space that was ice can now be food.) The dual battery can make your mind at ease of not worrying about killing the starting battery and it's super easy because these trucks had a stock option dual battery tray, so it's bolt on (seach ebay). A 100 watt solar panel on the roof will be more than enough for a decent sized fridge and some left over to run the stereo or accessories etc. when it's a nice day.
Regarding the heater: I've got one of the small mr. heater buddy propane heater that's rated as indoor safe. However, we only use it to take the edge off after dinner, late night swimming or washing up (assuming no camp fire) I would not run ANY of these in a non ventilated area while sleeping. The second you start ventilating too, you lose the heat advantage anyway. The real answer as Martin said, is a good sleeping bag or comforter/blanket. I don't know where you are in the country/world, but a good sleeping bag is 100% comfortable if rated to your need unless you're at mega extremes. Additionally, running anything that gives out enough heat to heat the inside of the car for a good period of time uses a TON of energy (whether electric or propane, etc., and become unfeasible, it would be more than your stock battery or one aux battery could handle w/ the small amount of solar you'll have). Use your own little heater inside of you, in a small enclosed space(sleeping bag) and insulate that

The car is just a giant semi/poorly insulated heat sink to the never ending heatsink outside, add one more step before you get there, and a much smaller volume
Looking forward to what you end up doing!
Edit:
Regarding the chair to beds: search "minivan chair" or "Odyssey chairs", there's someone who put honda odyssey captain chairs that fold to beds in a truck or van, may be a good starting point. IMO it may be a waste of space, as the chairs take up a lot of room. I think some type of cabinet or shelf in the back, that folds over for the third row may be better, may be able to sleep all of you on it. This will allow for storage and sleep area with minimal configuration time.
Also: regarding the kitchen, a platform like the above, can have some room to put a small sink with out of view pump that slides out of the rear of the vehicle, and has a spot to put a camping stove.. Between the frame rails or in the rear, you can have a water tank, IMO, they're hard to do right in cold climates where freezing is a concern. Search around and you will find TONS of solution, you can pick and choose the ideas, to what works with your space/ setup. I don't have barn doors, and not sure if you do, but if I did, I would put fold down shelving on the doors, so when they're open it could be a mini kitchen area for my stove