Putting together a "Peace of Mind Kit" is a matter of personal choice and availability, but ideally items should have at least TWO utilites to qualify for inclusion. The philosophy follows along the lines of - if you have to carry the kitchen sink, at least improvise on a drain plug.
Before offering suggestions for a possible expedition kit it should be remembered that in the event of a serious breakdown, the vehicle itself can be a major source of repair materials. Consider canabalising, modifying and using anything that is not essential to the vehicles safe movement.
OK, the list:
A 12ft tow chain, split into 4ft lengths with connecting shackles, is a piece of versatile kit.
Additional items will depend upon the terrain you're going into. If you expect to get stuck then full blown recovery gear like a winch or sand ladders will be needed. If getting stuck is just a vague possibility then sack bags, stuffed with what your stuck in, will often work. Messy though!
Finally, your greatest asset is your ingenuity - and it usually comes to you about 15 minutes after you've sat down and had a think!