honest opinion - do not do it because the costs would be alot higher than buying a better vehicle, the mechanics and there workmanship is unfortunately very poor except a few and those few will charge alot, hassle of finding parts and time required to complete.
After all of this is done by our normal mechanics, the vehicle might still not be able to serve the daily duty due to age and workmanship. There are many such examples on these forums that after a year or so of work, the vehicles are offered for sale.
This thread might give you an idea of the project but it was done by the owner and not by workshops https://www.pakwheels.com/forums/t/a-pajero-swb-1st-generation-restoration-and-upgrading-thread/176269/1
My 2 cents regarding your questions and we can discuss these further and there are many experienced members who would have better answers
What are the replaceable parts in the front suspension and steering assembly? The suspension is okay for now however the OCD and keera in me won’t settle until everything is done pristinely.
- suggest you get hold of a Repair Manual and it will give you all the details.
The steering pump leaks oil, should get it replaced or serviced?
- good quality new (not kabli) pump
The rear axle (probably – not 100% sure) makes considerable humming sound around 70-80kph when driving. Did have the rear axle serviced and the sound started after this – what could the issue be?
- what was done during the service and which grade of oil was used?
How much would the manual and auto gearbox cost?
- no idea
What things would I need to bring back the 4x4 system?
- v difficult to tell until done but 4x4 transfer case, front shafts and most probably a full service of front differential and hubs
5VZ vs 3C-t
- would recommend 3C-t out of the two but going to original 4d56tdi is not a bad or might be better option. Engine prices have recently increased, do check what they cost nowadays. Getting 4x4 system to work with 5vz might require additional parts.
Is there any more suitable route to go about the restoration?
- body change is not a bad idea and might be more economical
Costs of parts, contact of mechanics etc.?
- which city are you in?
Cost of Restoration
- do an estimate considering Engine, Electrical, Suspension, Steering, Interior incl seats etc, Brakes, Dent Paint, Rims and Tires. A very rough estimate is that it will cost above 500k but this is based on guess work.
Share more information and we can discuss further