Kimanchi,
Re kit, right, here goes. Basically, we bought everything we could for the trip (www. nomadafrik.com if you want the gory details) and subsequently drove through Africa like a beached whale. I should have listened to Chris Scott more (Sahara Overland) about weight. He's right.
Spares:
Think I covered that above. Wouldn't bother with anything heavy as you can always find second hand stuff. Do take a good range of gaskets, seals, belts and bearings. Grease and grease gun.
Fridge:
Well worth it. Engel or National Luna 40l both good and ruggard.
Power:
2nd battery and solar panel a good idea. We had 2 leasure batteries (Optima) as well as the main and would have much prefered a solar panel as much better for keeping the fridge going if you're staying in any one place for longer than 3 or 4 days.
A good inverter (True Sine) is better than one of the cheapo ones. Especially if you are charging laptop, GPS, camera, etc. off it. The cheap inverters really heat up your equipment batteries.
Lights: 12v strip ones from camping shops are good, as is a gas one for when you run out of battery power!
Split Charge system... Hmm, we had a National Luna one. It broke and we seemed to keep meeting people who also had one - and it was also broken. A switch, a relay and a good memory (to flick the switch) a much better and cheaper idea.
Winch - don't bother. It's heavy and they'll always be some-one to pull you out eventually. A rope, good shovel and sand ladders a good idea though.
Hi-Lift. Had one and found it heavy and a pain to use. Can be useful if you get really stuck in a ditch on your side somewhere. But you have a Series, so you'll be driving slowly and carefully. I'd say it's much better to get a 2 tonne bottle jack. Much easier for doing jobs under the Landy.
K&N air filter. Worth it I'd say. Don't think they do Snorkels for Series do they?
Fancy oversized wheels. I have 6 nice big 265/75 BFG AT's. Never a problem and never a puncture. But not sure I really needed 'em. Good tyres undoubtedly mean less punctures, but I wouldn't bother going for bigger ones - and one spare is fine.
Suspension upgrades? - hmm, drive slowly and you'll be fine.
Extra fuel tank and a couple of jerry cans - yep
Water tank - no. You can find bottled water everywhere. Lighter and cheaper. And the kids will love the empties. Also means you don't need a water pump/filter. Take a big 10l water bottle with you for washing up etc. They sell 'em in supermarkets in France/Spain.
Bull bar/roll bar. No need unless you decide to join the Paris-Dakar.
Security. Once you are in Africa, you're fine. It's getting through Europe that's the dodgy bit. Don't go overboard.
GPS - definitely a must. Also a lot better in colour, as we had a black and white screen that was a bit tough to see sometimes in the bright sun. Anyone will do really. If you're taking a laptop, well worth ensuring the GPS software works with it! (Garmin software doesn't work with Macs)
Sat phone. Waste of money.
Gas stove. Kettle. Mug. Yep.
Fold up camping table and chairs - yep.
Tent? Nice, light and cheap. There are plenty of places to stay on the way if you want. Our rooftent was fine - but a) it's expensive, b) it's a pain to put up and down, c) bloody heavy. If you've got a good roof rack - you can always sleep up on top.
Oh, and plenty of alcohol. Stock up in Spain.
Think that's most things. Sure others will have ideas as well. My overall feeling (as you can probably tell) is that you don't need half the stuff they try to advise you to get in the UK. And you're only going to Timbuctu which isn't that far.