Having never wheeled before, you and your friends will want to try and go a slow with your modifications, there is a world of difference in handling between a rig on 31" tires and 40" tires, kind of like the difference between a car with 100bhp and one with 500bhp, if new hands one is fine and the other could be deadly.
Looking at what you have to start with and your experience, I would build it this way
-Tires: 33 to start with. With the 2" body lift you have now you should be able to clear 33's with little effort, depending on wheel backspacing(or offset), tire width combo you might need to cut the front fenders or add an additional main leaf to the both sets of springs. An additional main leaf to the leaf spring pack will give you about an 1" of lift and wont stiffen the ride or reduce spring flex.
-Differential Gears and Lockers: 5.29 gears(will be a bit over geared for the 33's, but right for if/when you go to 35's or bigger)
Auto-locker front and rear, or if on the cheap spool for the rear auto for the front. Spool is better than welding, when welds break it takes out the gears, a spool runs about $125(U.S.), and is a better option than welding. Of course if the your funds allow it, ARB lockers are the best of both worlds.
-Body protection and recovery points:
Looks like you have the front covered as far as protection goes, so some rock sliders to protect the rocker panels will keep your doors functional, no need for a rear bumper, but you need solid attachment points for a recovery strap. If you want a winch, I would go with a receiver mount style with a drop hitch receiver in the front and rear so you can use the winch front and rear.
-Dual cases:
Based on the U.S. models you should have the cases you need to run dual cases(either stock gears, or regear one case to 4.7 gears) you just need the adapter plate to join the two cases together.
-Snorkel so you don't flood the engine with water when fording deep waters.
Without the stock bed you can go with a
flatbed(flat deck) to hold gear, or possibly something like what I did with what has been named a
Safari Bed if you are going to be out for more than just a day, or this
simpler version I rebuild/modified for a neighbor.
With what you have now(no mods) you could likely out wheel what your buddies have(depends on the model of the Patrol and Cruiser) if your wheling experience is the same, with the mods, I would think you would have the edge over them all.
With the drivetrain and tire set-up listed above, you should be able to wheel all of the terrain you stated, my only concern would be your version of a mud bog is. Here in the west part of the U.S. our version of a mud bog on the trail is little more than a puddle to our Southern counterparts, if yours is on par with the our Southern wheelers(swallows a 40" tire) you will need a lot more ponies under the hood than the Toyota diesel(a much larger budget than $4k), but then again with what your buddies are building, none of them will get through it either.
The double shackles were a bit of a fad here in the U.S., for the most part they are one of those ideas that looked good on paper/in theory, but in real world applications under constant changing input conditions they weren't. A buddy ran the "Revolver" brand, which worked fine on flat terrain, but when in an off camber situations the high side would unhinge, changing his center of gravity, increasing his risk of rollover, and after breaking one, he went back to standard shackles. The quarter elliptical design(part of a main leaf attached to the frame holds the upper part of the shackle) is a much better way to go than the double shackle, allows for more droop, but wont unhinge like the double shackles do.
