Hello Steve, I've been having these considerations for years now, and they go round and round.
Congrats on deciding to actually do it, that's a big step in itself.
I don't want to discourage you, but don't underestimate the time it takes to get a truck ready; Rob, a member of this forum who's gone to ground, built a lovely camper on an MJ but it took him much longer than he initially imagined.
The choice of base vehicle and the configuration of the vehicle depend on lots of factors, some of which are completely contradictory.
Fuel consumption: if that's a worry go smaller or get a new truck with a common rail engine. Most people cry NO to electronics but I know a family who have been travelling (Africa & now eastwards) since early 2003 in a nice new MAN without any electronic troubles.
Looking at the cost of a newer vehicle to reduce consumption:
If you compare a 10 MPG 8.136 with the 13 MPG 8.224 (what my friends run)...
At this stage I have to go metric: you save 7 litres per 100 km, if you saved 5000 pounds at purchase you have to drive nearly 143000km before the trucks have cost you the same amount of money. Do that sum with a spanking new truck and you've done a RTW before they cost the same (of course, reduced cab noise and aircon could be reasons to go newer, but they can be added)
Cruising speeds: improved by putting the biggest tyres that will fit, or going and hammering hard on a dealer (How are you getting on with that, Quintin?). And anyway only a problem in Europe, and once you're on the trip why are you on a motorway anyway? (Convoying with a TLC/LR can cause tensions sometimes)
Of your four candidates, as a (British educated) mechanical engineer I have developed a deep down mistrust of British manufacturing and build quality (remember the Leyland years? it took BMW a lot of work to get the LRs back up to standard) so for me the MJ and the Leyland are non-starters (sometimes literally ;-)
Have you considered Magirus? There are some inexpensive ex-fire trucks available in Germany and you'll find similar sized Magirus air cooled engines (& spares) all over the world (mostly in static generators). I presume you've combed Philipp Aus Dem thingy and mobile.de?
Going a little lighter I was intrigued by some of the ads I found at
http://www.dimensioneavventura.org/vendo.htm (there's a class A motorhome on a Daily4x4 chassis and a rare 4 berth Mog with cab & body rigidly joined)
and also
http://www.lt-4x4.de/ all mod cons in a less "in your face" package that's cheaper to run.
Big disadvantage of class A motorhome config: if ever you RoRo your truck unaccompanied, you need to block off access to the living area or risk losing the contents. Solution: take a cruise with the truck, you're on holiday after all (Grimaldi allow this, others?)
If my wife weren't hammering sense into me (and my weekends weren't filled with tiling/plumbing/roofing etc.) I'd probably already be mounting a recent(ish) MAN/MERC/RENAULT engine into a Tatra 813. Available in the Czeck republic for as little as 3500 EUROS, tougher just doesn't exist! A Renault Magnum 380 or equiv. with 1Mkm costs around 6000Euros and has all the plumbing necessary. (I'm told they get around 9MPG at 40 tonnes MGVW, the Tatra weighs 12T empty...)
Finding the space and lifting gear is another story.
I like Tatras! There, I've come out with it! Rigid chassis (= cheaper body mounts), protected transmission, 100T towing capacity (= tough transmission) independant suspension (= more stable on rough terrain), low CoG, I could go on.
She's right though, it would be very, very noticeable.
Oops! Lunchbreak over.
Let us know how you get on, what you decide etc.
Happy daydreams
er, sorry, preparations
Luke