Because the D2 is surprisingly unprotected with exsposed sumps and oil filter it only takes one stone for you to have a very bad day When you look at the jap motors they come with at least sump gaurds.Really it's just peace of mind.and the 5 P's

Ah, OK, a little surprised to be honest but as you say peace of mind. So worth investing some time and money I guess.

I have to chuckle really - I get all the arguments but keep thinking back to a trip through the Pyrenees - yes I know, not quite the same terrain - there we were all rigged up with big bad 4x4's and we kept being overtaken on the mountain tracks and trails by little Fiat Pandas ... :) who's drivers must have had a few giggles ...

Anyway - good travels - hope all the excitement is due to the scenery and not 'happenings'!

Dont take this the wrong way , but your not doing the Dakar . The discovery
doesnt need sump guards etc . The basic vehicle is perfectly adequate .
The only thing you might consider is track rod protection , apart from that
everything else is up out of the way . Remember you need to drive the vehicle home , unless you are setting out to try and do something like Ladoga JMHO

Friend of mine did Marocco in a Vitara and it worked fine. The downside was the drive from NL to marocco. Fuel consumption was bad, high speed was similar of a push bike and comfort was .. none. But once in marocco he smiled while I was suffering and stuck in the sand with my VW mkII Golf . He had to pull me out all the time. But it was fun for both of us :)

Fortune and Glory, kid. Fortune and Glory.

Not taking it the wrong way and I know I'm not doing the dakar.

But I have had brake downs when driving on tracks in both Namibia and in europe working as a gamekeeper spending 90% of my time driving on bad tracks.In landys and Jap car the landys had no protection so wher prone to being damaged so a little protection o the discovery will go a long way to reducing the risk,Apart from that the disco is standard and I am more than happy to go traveling any where in it.

I used to have a fiat panda 4x4 whilst in Italy so I know them well,they do get much futher than you would think they can but put some weight in them and thats it you are stuck.

you don't have to be doing the paris dakar to have underbody protection. it's just insurance. a lot of the tracks are very rocky and there's nothing wrong with having a bit of protection against possible damage. If people don't want it, thats fine, thats their choice, I've never had it either, but would have felt a bit more comfortable on occassions if I had it, but lets not give someone a hard time just because they want to protect their valuable vehicle that also happens to be their daily driver ffs!!

This is a really relevant dilemma for us...

We ideally want a Land Cruiser 100 series for the big trip to Aus, but can't help but notice a Hilux or Mitsubishi 4 door pickup can be had with a lot less miles for half the price.

Or for that matter, a low miles 4wd car we'd be happy to use as a daily driver into the city for many years after we get there, Volvo XC70, Audi A6 Allroad, BMW X5, etc , also low miles and less money by far.

Now we get all the arguments about a LC being the beast of choice for off road, but we really don't want to do that very much unless we have to. I can see it being immensely helpful in places like Mongolia, the Pamir highway, or Laos. But not really elsewhere?

So is it worth it to buy the offroad ability when most of an overland trip is planned to be onroad, or on rough tracks at worst?

A LC isnt mandatory , a "lesser" 4wd would suffice , but some 4wd are not that ideal for the trip. eg the X5 , mainly from a tyre /wheel point of view.
Going further along the cost/capability line, then a van with reasonable ground clearance would do just as well. The 4wd thing can be a must depending on what you intend and roads/tracks you wish to traverse at what times of year . You also need to consider the tech level /fragility of the vehicle . HTSH

you'll only take an x5 off road once!!! honestly, most 2wd cars are more capable!!
Parts availability and having a vehicle familiar to local spanner twirlers is also important, and on that score it's hard to argue against a hilux. they are known all over the world, more so than landcruisers and landrovers

Just keep in mind getting parts for some of those other choices might be difficult in some remote areas, where as LC parts you can generally get anywhere.

I couldn't possibly recommend you take an A6 Allroad, unless it's very new, and I work for Audi! Suspension problems galore on older models. X5 won't be any good.

Volvo XC70/90 surprisingly not bad, quite reliable too just poor fuel economy and again the issue with parts if you do break something you will be there for days just waiting for parts.

LC is a great overlander, it's actually a great vehicle in general for anything, just pricey. As you mentioned you can get a Hilux for a lot cheaper, but I would say don't shy away from mileage/KM too much on an LC as long as it's been maintained properly and you check it out thoroughly and drive it a bit before you begin any big adventure it shouldn't give you much trouble.

Older XC90's have some reputation for gearbox issues which could be awkward in the boonies.

I think also with the newer cars you ned to think about fuel quality in some of these places.and also taking a diagnostic tool so you can find out what the problem is and wipe errors if it's just some random fault.

This is true of true of the locals all over the world, and it does make you feel like a plank at times, but in Morocco for instance, when we're out playing in the dunes or cross-axled in dried up wadis, there isn't an 'African taxi' (a Peugeot 504) in sight.

Most 4x4 owners I know drive them because they can go places that ordinary cars can get to, like mountainous regions, and with all the skill in the world a two-wheel-drive car can't climb without running out of traction (especially when it's loose terrain or wet/icy) ... not to mention puff!!!