I don't want to confuse you even more, but I would certainly consider the Garmin GPSMap 278 as well. This is basically the same unit as the GPSMap 276C BUT it has following advantages :
Build in memory of approx. 2GB, preloaded with road maps for all of Europe. The 276 only uses Garmin memory cards, which are expensive. ( if you want, you can erase the maps you don't need and put other maps in its place )
Takes advantages of the POI ( = Points of Intrest ). This means you can have literally 10000s of points loaded to the GPS ( most common use is speed cameras, but this can equally be camping spots, fuel stations,....) These can be found free on the internet or you can make them yourself.
Even faster processor.
The first 2 advantages are really worth having for such an overland trip.
BUT if budget is tight, a Basic - non mapping GPS will do fine ( especially in Africa ).
I used to live (and work) in Tunesia and used a basic GPS ( these where the only ones that existed at the time ) together with military maps on the computer. I had a nice program running and made all my routes on the computer and transferred them to my GPS. This worked perfectly.
In Europe this would not work as there are way to much roads, but in Africa : No problem.