If the base vehicle is this bad why are you starting with it? Is it an attempt to save £200 a year?
This overlanding thing seems a bit like an exercise in how many kitchen sinks I can carry across the Alps without my Land Rover once going wrong, you may as well use a vehicle from another brand, something like DAF for instance!!
How on earth did we manage in the past with standard Series vehicles? There does seem to be a culture of "you NEED this" or else you might die or something ridiculous
Frankly the best vehicle for the job is the Discovery that you are trying to immitate with a Series body on top! I can categorically say I have never been overlanding, but as someone who spent long periods travelling the length of Europe in a 2.25d 109 with my old man, and whose friends drove to Switzerland and back in Series 1s with nothing bigger than 1997cc under the bonnet, I will say you can go and over complicate things.