Hi Dave,
I'm a Yank too, but living in the UK. I have a 1984 (A reg!) 110, with the 300TDi, R380 5 speed and a lot of other kit from later model years. I believe in the lingua franca it would be described as a "Bitsa" -- bitsa this, bitsa that... I bought an early truck with specific purpose of being able to take it to the US some day.
I have a friend in Oregon who converted his RRC from a 3.9 to a 300TDi. He saw the mpg get double or better. The 3.9 was getting 12mpg (US gallons, smaller than Imperial), and he sees 25+ with the 300TDi, with the 4HP22 slushbox. It accelerates slower, and it won't top 85 mph, but it will cruise at 70 all day.
My 110 gets in the neighborhood of 20 mpg Imperial in mixed driving.
A few things to consider if going from a petrol (gas to you) Disco to a diesel 90 (from experience, since my prior vehicle in the US was a 3.9 RRC):
1. Noise -- lots of it. 90/110/Defenders are much louder than the passenger-focused Disco, and the 300TDi is not a quiet powerplant. You can tone this down with sound deadening material, but a 90 is never going to reach Discovery-esque levels of NVH.
2. Speed -- the 300TDi is a lot of things, mostly good, but fast is not one of them. Acceleration is (as Rolls Royce used to say) adequate, and you will be able to cruise at 65-70, but unless you spend some money on upgrades, the top end will be about 85. Personally, I think 65-70 is plenty fast in my 110, and I have 20 more inches of wheelbase to keep things from getting interesting in curves and crosswinds.
3. Space -- the driver and passenger in a 90 have plenty of room separating them, but in all other directions you are rubbing up against truck. The door is rubbing your right (or in your case, possibly left) elbow, your head is a couple inches from the B pillar, the seat is close to the wheel, the dash is only a few inches from the steering wheel. In a RHD truck the parking brake handle is where your left leg wants to be; that can be fixed by swapping to a RRC handle mounted between the seats. In a LHD truck the transfer case eats 4 or 5 inches of the footwell, and it's going nowhere. If you are over about 5'10" or have long legs, you will want to put spacers under the seat rails to relieve the sensation of "squatting".
4. Comfort and modcons -- New defenders have heating, defrosting and ventilation that would charitably be described as satisfactory. Older trucks are simpler and will prevent drowning or freezing, but only just. Door and window seals will slow down the wind and water, but mostly serve to reduce rattling noises. The heater works OK in the front 3 feet of the truck, beyond that the temperature is dependent on local weather conditions. The defroster setting will keep the windshield mostly clear, but if it's very humid or cold, the side windows will fog or freeze. The wipers are just OK. You can get A/C, but you'll give up about 3 inches of the limited space in front of your knees for the privilege.
That may sound like an attempt to scare you off, but it's not. I love my 110, with all it's "personality". I have no intention of ever letting it go.
Some (not all) good things:
1. The 300TDi is a granite block of reliability if you keep up on preventative maintenance. Try to find one without the ECU-controlled injection pump for the ultimate in simplicity -- The engine can be made to run with no connection to the electrical system save a battery to get it started, and in truly dire circumstances you can push start it.
2. Defenders are like an adult Transformer toy. Parts will interchange from as far back as the Series III, probably even some from the Series II, and it's all bolt-on, bolt-off.
3. It's a real Land Rover! (cue leaf spring chorus of disagreement...)
