[mr48chev]

That is a good looking old survivor.

I'll try to answer your questions as best I can.

Converting to 12 V isn't that hard but you want to make sure your wiring is in tip top shape. The same things that would be required to change over a car from the UK to 12 volts apply on it. Changing all of the bulbs and putting a voltage reducer in line to the gauges. It would primarily be the fuel gauge on that one that needs the voltage reducer.

They are comparatively slow by modern standards but how far do you drive your load carrying truck at a time and how much of it is on roads that allow speeds above what you can expect from the 48 . I'd think 45 or 50 is probably about what you can comfortably expect out of it on a regular basis. Remember that you have to be able to stop as well as go.

Diesel engine, I don't know if they have the right injection system but those Isuzu NPR box trucks seem to be world wide and a lot of guys here are pulling the engines out of them and putting them in pickups and similar trucks. there may also be a Perkins engine that is/was sold for similar size trucks made in the UK. Parts availability would be the key along with reliability and enough power to do what you need.

On the hydraulic pump, there are a number of ways to go outside of running a PTO driven pump as it most likely has now. There are belt driven pumps along with the electric pumps. And don't leave out a pto on a newer trans.

I have no idea about parts sources there.

Disk conversion, again I have no information. later big truck disks could probably be swapped if you have someone who is an excellent machinist who can modify the spindles or hubs to adapt one to the other and then fabricate caliper brackets.
I've been toying with some ideas for redoing my 1-1/2 ton 51 Chev's suspension but that is many projects from now before I get to it.
Welcome to the board, I should have said that first.