You don't have to change a 24v to 12v there is no law that says so. You would have a bit of a nightmare trying to get bulbs and stuff for it though.
It's a 1991 so it is unlikely to still be 24v (that is assuming it ever was). In fact looking at the picture it has plastic side light and indicators which would melt with 24v running through them, so I'm 99.9% sure it isn't 24v.
I may be leading you down the wrong path, worrying about 24v / 12v. It is just in my experience we bought a FFR which had been converted to 12v by a blind cowboy in boxing gloves. Best bet is to look at the wiring under the bonnet and under the passenger seat (if the battery is in there), if it looks like a bowl of super noodles you are likely to have issues. The Landy is 22 years old so it WILL have had things wired in here and there over the years, whether it was ever 24v or not. You are looking for shoddy workmanship, any sign of it start to worry.
Ask him why there are 2 fillers, it may just be that it has a secondary tank. Lots of Landy owners fit them.
The guy has told you it needs new UJs, these cost £15 each and take about 1/2 an hour to sort if you know what you are doing. Symptoms of ruined UJs can be the entire vehicle vibrating like buggery, that could also be an out of balance propshaft which is a specialist fix, not expensive but can take a while. I'd ask him why he has not sorted it. If a propshaft has a ruined UJ you can just remove it and run in 2 wheel drive.
The 2 main things to worry about are the bulkhead and the chassis, if either of those are gone you are looking at scrap. If you have any mechanical knowledge you will find that everything else is easy enough to fix. Oh, and if (when) you need parts look
here first.
