Lets start with modifications and work back.

Off the top of my head there are probably only four modifications that I would see as semi essential

1) Good quality wheels x 6 - if the vehicle doesn't already have these then change to a good all terrain or mud terrain. I would always choose to have 2 spares. If a tyre is badly damaged, blown out, hub damaged it may not be possible to repair or easily replace.

2) Good suspension - you can approach this two ways, either upgrade the suspension to cope with an increased expedition load or alternatively keep the load down (cyclists manage to cross Africa with almost no kit so one could keep it light). However don't load up but going on lower suspension, it will probably break sooner or later.

3) Cargo barrier - I think this is often overlooked and people are really looking for a very bad outcome in the event of an accident. With masses of heavy kit in the back all coming forward, the barrier can make the difference between a straightforward shunt with some vehicle damage and a life threatening situation.

4) Dual battery system - its easy to flatten the battery over night.

Other than this, I would regard pretty much all other mods as either making the vehicle more comfortable, more capable or just for looks but you don't need them on 95% of expeditions.

In terms of kit to take with you, you can cover the fuel range by using jerry cans, nice to have it in long range tanks but certainly not essential.
2 decent shovels, at least one with a long handle to get under the vehicle, air compressor, wood saw, recovery strap, tow rope, tent, camping gear, etc. If going to be travelling solo then maybe a tirfor for self recovery. a section of chain, hi-lift jack, etc. Other than that several books such as Haynes manual, sahara overland, Tom Sheppards overlanding guide, etc. You can usually effect a recovery with the right knowledge even thought you only have limited kit. eg, using a hi-lift as a winch.

In terms of acquiring a vehicle, I would be inclined to look for something that isn't modified, not because I don't want the modifications but because most modified ones will have been used in ardous conditions, either overseas or offroading etc. Obviously if you can find one that has been just driven round like a normal vehicle but for what ever reason has all the kit then great. As most of us know, doesn't matter what you spend on accessories, they aren't worth much second hand so nice to acquire but would much prefer to start with a reliable vehicle than one that has been used heavily offroad or overseas where it may well not have been welll serviced, etc.

I would go for something like a N / P reg 110 Defender 300 TDI in good condition with a good service record, then start modifiying from there.

One option if they have the cash is maybe to buy two vehicles, buy the first one with all the accessories, strip it, sell it off and then fit them to the second vehicle. This way you get a reliable vehicle with a load of cheap accessories.

If you just buy a vehicle because it has all the accessories but may be has spent lots of time in hard conditions then you will probably spend lots of time reparing it.