Welcome bud, from another Trol owner (mine is a '54 plate so about the same age). The Y61 Trols are very capable and are comfortable inside so make good overlanders.
As said, first priority is a decent set of tyres, assuming a mix of onroad/offroad time get some all terrains. There's a lot of different brands we can recommend and each of us have our preferences! :lol
Event ML 698 are cheap but do the job, bees knees (but expensive) are BF Goodrich AT - they're very good and will last ages. Personally I'd give a shout for General Grabber AT, look a bit unconventional but no complaints here and were less than 50% worn after 20,000 miles. There's been a lot of posts on tyres over the years, might be worth checking the search function
While on the subject of tyres I would also suggest you go up a size as Trols the age of yours and mine are only on 31" tyres as standard (265/70 R16) but the later models were sold on 32" without there being any mods to the suspension etc of vehicle. I've got 265/75 R16 muds fitted and have no problem with rubbing/catching and it's an easy way to give yourself a little bit more clearance (albeit not massive).
Double wheel carriers are handy and look nice but to fit one you're probably looking at having to get a rear steel bumper with one incorporated (£££) so that the weight of the tyres isn't all on the rear door. Cheaper option would be to get a roof rack and stick a second spare wheel on it or stick the wheel in the boot. The only other benefit of a rear steel bumper is that it makes the indicators and rear/brake lights less susceptible to damage but it's a lot cheaper and simpler to just buy a set of euro spec rear lights which then move the indicators etc up to the rear boot pillars (very easy to do as almost entirely plug and play - I did it on mine and my write-up is on the forum).
CB might be useful?:nenau
I would also suggest fitting an oil pressure gauge as the high oil pressure switch (there are two, a high and a low) on the 3.0 has a habit of playing up and putting the oil pressure warning light on even though the pressure is fine. You can replace the switch but it's not the easiest job to do or access and so a lot of people (including myself) just fit an oil pressure gauge instead with the sender plumbed into the much easier to access low oil pressure point. Having the gauge means you know exactly whats going on
Any other mods like a winch bumper, suspension lift etc depend on how offroad you want to get! :thumbs