Thanks for the input and yes I'm trying to make a choice between either the lighter weight M416 or the heavier M101...or barring either of them, having a trailer base fabricated. My tendency is to go with the military trailers because they can be pretty much ready to go depending upon the mods you want to make.
The intended use is for two people doing a couple of 3k-4k road trips with some off road every year plus the usual weekender camping trips. The Discovery is a heavy old beast and can slow down on hills all by itself, but otherwise is rated to tow 7700 pounds braked so I believe it can handle an M101, it just won't be very fast. That's OK since I'm not in a hurry.
I'm going to post this question on a couple of Land Rover forums and see if I can find someone who has an M101.
In the meantime, I have another question since I'm still trying to educate myself about trailers and suspension....it seems like lots of owners of military trailers replace the axles. Is there a particular reason why?
I tow my camper with a 6 cylinder, and have a ton of mods on the truck. The 4Runner is rated for 5000#. The trailer is about 2600# fully loaded (and literally includes the kitchen sink) and it pulls really well, I'm just not using overdrive very often. I think a good start point is what your creature comfort level is...mine is not a minimal approach.
As many have stated the trailer becomes your base camp for the days travels. If your looking for something is to be more of a "camper/teardrop" or just a gear hauler? Part of that equation (at least in my world) is the set up has to comfortable in any weather conditions..the down side of that is that is the extensive set up time. I usually only take the trailer out for 4 days or more, unless I have help setting up and taking down the tent.
We share a similar philosophy...I'm rarely in a hurry to get there, and want to enjoy the ride.
The military trailers (my favorite is the M116A2 flatbed) for the most part are extremely stout, and unless they have been purposely abused, are pretty much indestructible. The same is said on the M101A2/3.
Electric brakes and to have a common lug pattern with the tow vehicle.
One spare fit all.
As Mark states here, the axle change, if ordering a 3" diameter (5,000#), for the most part can be a direct bolt up. You do have to cut off and weld back the spring perches, but outside of that it was easy to do. I ended up using aluminum Spider Trax because my wheels are hub centric, not lug centric, again, an easy mod....and you can share the spare between both.
If your approach is more minimalist, then the M416 size (Adventure Trailers/Manley) will probably be more than adequate.
Good luck with your search!
Pat