The low kms on that truck, if genuine, would make it an attractive proposition, depending of course on the price - how much $$$ is the owner looking for?
Given you use of the word 'petrol' and the scarcity of the FJ45 in the British Isles, I would suspect you live in Australia - correct?
And yup, the 2F is likely to return you, in good operating condition, somewhere in the range of 12~14mpg.
A diesel-engined version of the FJ45, namely a BJ45 or HJ47/47, will give you near enough to double the fuel economy of the gas engine, typically 20~24 mpg. Swapping a diesel in can be relatively painless, or a major project, depending upon which diesel you swap in and how many changes are required to make it work properly. While there are lots of diesels out there, some are more suited to trucks than cars, some are too big and heavy to be put into a cruiser without serious suspension or sheet metal changes, and some are difficult to tie into the remainder of the Toyota drive train. The easiest swap for the F type of engine would probably be the Toyota H or 2H engines, as they can interchangeably bolt to the same transmission and take up, in the case of the H/2H, the same rough amount of physical space in the engine bay. I wouldn't bother with the H engine though, given a choice, as the scarcity of parts for that engine would present a hurdle. The 2H is still an old engine, in production from 1980~1990, and made it into many markets. Parts are still available, though there is a sunset not too far off for parts availability I'm sure.
The Toyota B series of engines are great, however a lot of mod will need to happen before you can bolt one in. It would be best to get a hold of a 13BT (the turbo version of the 3B), complete with 5-speed. In the non-turbo B engines, a 1985 or later 3B has the advantage of an extra bearing for the camshaft, so is a touch better than the earlier models.
Now, if you are real handy in the fabbing department, you might want to consider a sweet newer Toyota engine like a 1HZ - gobs of torque, very quiet, low emissions, parts not an issue at all (unless you live somewhere that never received these engines into the market, like N. America).
If you get into diesels, you may also want to get into biodiesel/WVO at some point, and with those types of fuels it is better to run an older diesel with the indirect type of injection system and inline injector pump. These are better suited to running alternative fuels than the rotary pump/direct injection types. Pretty much every Toyota factory turbo engine (13BT, 12HT, etc, uses indirect injection. In fact the use of the number '1' prefix in the engine model number, designates direct injection type instead of indirect.
The normally aspirated diesels, like the 2B/3B/2H, can all be turboed with their older type of injection pumps and indirect injection. The B series are a little better suited to turboing, as they came with factory piston skirt cooling nozzles. This is no a big issue though, as there are brands of hi-temp compatible pistons available for the H type of engine that don't require piston skirt cooling. Also, the piston cooling oil nozzles can be retrofitted to the H/2H series engines without too much fuss.
Turbo engines develop higher cylinder head pressures, so it would be *best*, if you modified a NA diesel with a turbo, to change pistons out to drop the compression ratio. By way of comparison, a NA 2H is around 20.5:1 compression ration, while the turbo version of that engine, 12HT, has a compression ratio of 18:1 or thereabouts. This is just from memory, so my numbers may be slightly in error, but pretty close.
You never mentioned whereabouts you live so I can't give any more suggestions as the focus is a bit uncertain.
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