Exactly Frank -
You took me litterally with no limit assumption. You too know that a 24L tank is far more prudent than a 18L and a Huskey tank is more prudent than a stock 24L - when going into the bush. I do however prefer different containers/canisters = if the bike smashes one, there is a second spare. Limit for petrol is; a bike that can be handled - I'd say somewhere around 30-40L even though there are tanks on up top 45L (if I recall correctly).
As for water - usually I see people bring a pint (0.5L) to larger Coke (1.5L). This is ok when on ordinary roads with shops and people around the corner so to speak - but when you're going off into the bushes so to speak; bring water in a supply that covers at least a couple of days (ca 5L/day).
As for the other recommendations:
changes, shelter, spareparts, etc - can be bolted on the bike or put inside panniers; I prefere inside panniers = dry and keeps bugs and creaps out as well. I never wrote -fill- the panniers... did I -
Panniers also have some other aspects - protecting the legs when falling over, gives leverage when needed to lift up, pull and push, frame stabilization. Easy to lighten the bike when need be - just take off the panniers; if it rains does not matter for the content; if tied to the bike => more work and can not be placed un a wet and muddy ground less one accepts even more work...

Yes - ALL extra stuff weighs, and ALL extra weight is something one swears madly about when stuck somewhere!!! I ride at winter - the opposite conditions to Wadis planned tour. I used to tie down the canvas cover to the bike on pillionseat - now I used the top-box to pack the cover in. I allways carry a 5L petrol can when on longer rides - does not matter if I use that petrol; it is my "in case of" spare volume. I also always carry a spare snowmobileoverall. Thats two (2) panniers filled and the extra weight is about 2-3kg merely. In africa - conditions are vastly different; opposite so to speak; but in africa there is heat, rain, bugs and creaps instead.

Of cause he shall load as lightly as he can -
Petrol and water are really essential - that is where I recommend him to focus on enhancement options.

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I did not adress spare choices - I drive /7 modells. I did recommend Wadi to look-up what the 1200 is prone to need. Planning a long drive/ride - demands full planning, hence my approach to trigger this very good and elusive discussion that have emerged -
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Alternator = Generator ; may differ between languages. Yes - stock car regulator/rectifyer - and it is a really expensive one to that is used on the modern Bosch "electric production units" as compared to the older. About 100USD (!) as comapred to about 10USD. Two major options available; Bosch and Hella - I buy patterns.

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Drive Safely,
Albert