Good question. In the last ten years, I have done a good few of the off-road and overland tracks in Australia. Sometimes in cheap vehicles and sometimes not. I will put aside the questions of route, spares, tools, first aid so on, and how severe you have planned you're route for broad suggestions. There is loads of info on that, and a lot is just personal choice and what you get used too.

I have found the best mods to be "really necessary".

1. Tyres
2. Suspension
3. Snorkel (good for water crossing. Even better for dusty conditions)
4. Long handle shovel
5. Driver Training (the none sexy stuff, but you will thank yourself later)

 Sand flag (is a legal requirement in the Simpson desert) but depends on where your route takes you
 Compressor, airline and pressure gauge, really useful for soft sand so on
 Rated shackles, snatch strap, and good quality recovery equipment

o Sand ladder (max tracks) – used once, they look cool
o High lift jack never used it, and they also look cool but can be very useful, especially for lifted vehicles. Be careful with them!

o UHF is like the winch and sat phones, essential when you need it and useless the rest of the time. I'm still waiting
to use them, and I hope I don't need too. You make your own choice.