Per the design spec a 101 will go to 45° tilt and will climb and descend a 60° slope. With the designed load in the back you have a near perfect 50/50 weight split front to rear. They are not rock crawlers and were not designed to be but a GS is at least as stable as any other rover off road. All the weight is low down in the chassis, gearbox and engine with very little of anything above the height of the load bed.
As you point out the 101 was designed to haul a cargo and is therefore sprung for that instead of the typical comfort ride of a dual use vehicle. With anything upward of about 400lbs in the back the rear suspension starts working nicely and the ride becomes a lot better. Contrary to popular misconception you end up with about the same wheel travel as a stock 88.
The ambis already have a good amount of weight on the back so I would not bother respringing it for a camper. If you plan to run a GS lightly loaded Paul Haystee in Holland has produced a set of springs for the 101 that soften the ride considerably.
For campers most either look at radio bodies which are more expensive and rather small or go straight to the ambi which tend to be cheaper and have a considerably larger rear body with ample room to live out of. There is also a gentleman in Canada who built a camper pod to drop onto the back of a GS in its stripped form. It was for sale fairly recently.
With all that said you do pay the penalty of it being a military vehicle with very little to no concessions being made to comfort. There are disc brake kits out there and power steering is a common conversion.
You would not want to put something as heavy as a 6BT in a 101 for the very reason you state as the extra weight makes them very nose heavy. The commonly accepted wisdom is to use a 200/300Tdi or 2.8TGV. Nissan diesels have been transplanted with good results. Some have put auto boxes in but that means you cannot have the big Nokken PTO winch.
There have been some 6X4 conversions of the 101 but very few are truly successful. By the time you have stretched a 101 and done all the rest of your mods you would simply have been better starting with a 130 or taking a step up and starting with a Mog 404 radio body. Land Rovers are big Meccano kits but at some point you have to lok at the expense and effort of turning a vehicle into something it was never designed to be vs starting with a more suitable platform.
If you are really interested in this the 101 club site has an excellent forum that has threads covering most of these topics.
Finding 101s is starting to become more difficult even in the UK but hardly impossible. Even with a fair number of failed projects out there finding a chassis cab for sale just does not happen and really wouldn't affect the value compared to just buying a GS/ambi and stripping it of unwanted parts.
On the other hand you can build a really nice beach runner as shown on the ECR site.