I suggest the 2500 and the 6.5 Diesel
I have a similar rig and highly recommend going with the 6.5 turbo diesel to help with range. I'm consistently getting 19 or 20 mpg on the highway, whether I am pulling a single axle utility trailer or not. The diesel's abundant torque makes towing a breeze. I made a few mods to enhance reliability -- relocating the PMD and upgrading it to one that better suited to handle heat, and changing the thermal actuator on the front diff to a mechanical one that actually works in deep snow). I consider these to be the essential mods for a 6.5 diesel on the 2500 Suburban. I found one on CL that came with a 4 inch lift from Trailmaster, which I believe makes a 4 and a 6 inch lift for this vintage Suburban. Word of caution, Trailmaster is running out of replacement parts for this lift. You could check with them to confirm, but there may be better lift options out there, as some have mentioned.
Personally, I think a four inch lift seems just right for this vehicle--a good balance of height and ground clearance while not totally sacrificing fuel economy and front end wear. I am running 295 (33.5") Toyo MTs on 17 inch rims with 4.10 gears, which for me is just about right. I wanted to have plenty of room in the front for running chains without having to modify the front wheel wells. I have had zero problems with the IFS but I have upgraded the ball joints and tie rods don't beat it when driving offroad. [I did enough of that in my twenties when I didn't mind walking out 5 or 10 miles to a main road and flagging down a ride.]
For your range needs, FellowTraveler told me about a guy outside Portland who still makes an auxillary fuel cell for your desired rig. I talked with him recently. He doesn't have them in stock, but he can build one in a week or two to fit your specifications. Check out his 25 gallon mid-ship galvanized steel tank here:
http://www.nwmp.com/index.php?option...-gmc&Itemid=17 With your 42 gallon stock tank, plus this 25 gallon transfer tank and two five gallon jerry cans mounted on the back or on top, would get you to 77 gallons, which combined with the 6.5 TD's fuel economy, and sane driving habits, would give you the range you desire.
It would be mounted mid-ship, driver's side in front of the rear axle. There are spaces on the other side of the frame on that side, or better, the passenger side, for mounting smaller items such as onboard air systems, a third battery, a water tank and the like. I would be interested to see how creative suburban owners have been in frame-mounting accessories such as these. I've got some plans of my own to experiment with different onboard air and water tank options.
Good luck with your plans.
DMT