Do not buy wheel spacers. Big, offroad E-rated tires are a crapload heavier already and put a ton more stress on all your bushings, ball joints, and axles. Wheel spacers add a ton more stress to all those parts. Do some research on performance driving, both off and on-road. No one looking for function + reliability will ever install a wheel spacer.
I re-ordered your list, with some notes.
Required, Required, Period.
1- IID tool from GAP Diagnostics ($550) - Best money spent, hand-down, on an LR3/LR4 if you don't want to go broke at the shop/dealer. This should be bought along with any truck.
Required for even moderate off-road duty:
2- Tires & wheels - Definitely do your research and as everyone has said, go with 18s and drop the money on wheels. Compomotive PD1881 or Lucky8 steelies are the general go-tos.
Outstanding and nice-to-have if you camp a lot
3- Dometic CFX40 fridge ($500) + 2nd battery ($100) - Do your research. Unless used, a decent fridge is twice that much. Also look at Snomaster for the best durability.
Optional for moderate off-road duty. Good common sense can avoid the need for these in most cases
4- Rock Sliders ($350) - Good sliders, as mentioned need to truly protect. Don't skimp on cost. Buy quality sliders which will hold the weight of the truck.
5- Maxtrax ($300)
Not worth the money unless you do a lot of heavy, technical driving...or just have no common sense
6- Bumpers Tactical front ($1720) and rear ($?) Lucky8's - Tons of weight which will wear out parts faster, but definitely functional if in the back-country a lot. Just keep an eye on your air struts and compressor performance.
Completely a nice-to-have.
7- Rear Differential Lock ($1500) - The traction control in these trucks is more than sufficient. If you truly need the rear locker, you probably bought the wrong truck. Also, no way would you do this for 1500 unless you source from a junkyard and do 100% of the work yourself. There are zero aftermarket options for the factory rear diff (only ARB offers one for the front).