I'd say that the weight of the vehicle would put you around fully loaded Burb weight...so look to see what kind of mileage the 6.5TD burbs get and probably drop 1-2MPG due to the full time 4WD.
I'd chose the 92 as it's mechanically injected, which simplifies the swap but you will have to figure out how to make the sensors work with the factory gauges, if its possible. You would also need a transmission controller to control your factory transmission. I think you would run into more issues using the Electronically injected 94+ 6.5TD's, which are far from simple.
I think you're gonna need to buy a new flexplate and torque converter to fit your input spline count (not sure on the 97+ transmissions), you should be able to use the factory 700R4 flexplate (these are externally balanced and have to use a specific plate) and get a custom converter built for the bolt circle of the flexplate and fit your transmission and the stall speed. Otherwise the transmission can be adjusted from the TCU or through a custom tune for shift point and other variables.
The 6.5TD can be modified to run pretty good, but the best you can hope for is for first gen D-Max performance. You should be able to obtain 14-16MPG consistently if you drive it right. It won't be a speed demon, but with a healthy IP, high pop pressure injectors, plenty of air coming in and lots of space for exhaust to leave and it shouldn't be far off from the performance you get now. If you've got it running right, tuned to perfection, beefed up bottom end and gauges aplenty, you should see close to 250HP and 500TQ and handle more than stock limits and be nearly as reliable.
Just don't get your hopes up! but it sounds like you know what these engines are like anyway.