Actually I can't remember what usually goes wrong on those despite owning one for 4 years - it never worked often enough for anything to really break. The best accessory I would have fitted in between trying to get it started would be an interior light!
Now let's see,
if it were a Disco you'd need a welding kit.
if it were a RRC, you'd need a welding kit.
if it were a P38 you'd need a new engine and a multimeter
if it were a freelander you'd need, ummm, a 90.
If it were a Defender, you'd need ear defenders.
Can't think of anything for a series apart from a bulkhead removal bar for tall people but that's already been suggested. AA Membership?
That about right or have I missed some?
Guy