Lots of good advice coming out here. Definitely agree with the US Dollar advice - but make sure any notes are year 2000 or earlier - it used to be 1996 notes and older that would be rejected, but now it is also pre-2000 notes that give problems. So new notes are essential. Lots of denominations too - or you will 'lose' money on change not being available.
As for the route, you need to do the research on where there is a reliable Avgas source. Khartoum, Luxor and Cairo have fuel but check its there when you need it. A friend had a problem coming through Egypt and when he landed, the Avgas wasn't there on that day "but was coming soon". Cost him a day's delay.
Eritrea and Ethiopia aren't friends, so transiting between one and the other is sometimes a clearance issue.
There's lots of very dry, very empty areas to overfly - make sure you have water & provisions for all emergencies. You might also be out of radio coverage with anyone useful for long periods, which upsets many people. Usually not such a problem with ATC as long as you make contact when you can.
There are a number of restricted areas on this type of route - no overflying presidential palaces, dams, reservoirs, military training areas etc It really upsets the local government/police to have small planes down low over such areas. So check these out before you get in trouble.
Epaulettes around airports are extremely useful passport to get through most issues. Visas and appropriate overflight/landing clearances definitely!! And lots of General Declarations/Manifests to hand out like confetti to any airport official that requires them. If in doubt, give out a manifest. Some places require six or even 8 copies before they're satisfied.
Weather: We're getting towards the rainy season in East Africa. This can be dramatic thunderstorms but generally short lived CB's which you can mostly get around or delay to avoid. Its not frontal weather but isolated or embedded storms, but not to be messed around in a small light aircraft. They reach 40,000' and build really quickly so don't plan to go over developments - go around. The major weather phenomena is the ITCZ which is in South Sudan/North Kenya heading south at the moment. Low level jetstream can be your best friend in these areas - adds 15kts sometimes if you catch it.
Avgas in north Kenya is not readily available. Fuel supplies can be few and far between. Alternates are distressingly far away when things go wrong. Another friend thought he had fuel arranged in Lokki through MAF but it wasn't the case. Until he got it sorted, he was in quite a sweat - there's no where else to go!!
What else do you need to know? Ask away - I'm sure there's lots of useful advice out there! Good luck with it - a great thing to do and worth lots of memories/photos for the grand kids....