First thing is, do it, you won’t regret it. Few things to consider, is it cheaper renting an overlander rather than prepare your own vehicle.
I have a Defender 130, and preparing any vehicle for multi day trips where there is going to be off-road driving involved takes careful planning.
It’s very easy to chuck money at your car/truck and not use any of the expensive add on’s that people insist you must have. Examples, you don’t need a winch if you have a shovel and a high-lift jack, even in the deserts of North Africa (some Max Trax are very useful though), you do not need multiple jerry cans or a fuel tank extender if you have planned to never be more than 200 miles from a fuel stop, same goes for excessive water storage.
Contentious one, you don’t need a roof rack! Why not use a ground tent, storing kit on the roof adds weight in the wrong place, plus it is poor for your fuel economy.
Iceland involves water crossings, so a raised air intake with extended axle breather pipes is a must, even more of a must is learn to drive the ruddy thing…plenty of UK courses available that allow you to use your own vehicle.
I could go on and on, but plan carefully and scratch off most of the kit you think you need.
Drop me a message and I can expand some more on the above.