Steel is better than al for touring. It is less likely to crack, the rack attatchment threads are tougher, the frame may be a bit flexier but this make it comfortable. Al touring bikes are very expensive and steel, even budget level steel has a good reputation. You can repair steel using traditional brazing techniques common in most towns and Africans are renound for their ability to fix stuff.
Modern tourists are returning for steel as material of choice for expensive luggage racks, although Al is still worthy.

The difference between budget bikes and entry-level proper brand bikes:
1, materials: branded bikes use better grades of steel and even where they dont, the tubing is more reliably made.
2. Design: Proper bike designers making proper bikes rather than wannabe MTBs for box-shifting stores. They have the full complement of threaded attachment points vital for relaible touring.
2. Construction: The joints are better prepared and better made so are stronger and more reliable.
3. Alignment. Budget bikes are notorious for having their wheels point in different directions.

Size, fit and saddle are all vital for comfort.
Most African tourists use traditional pedals rather than modern clipless ones: Clipless pedal systems are expensive and when they break you cannot pedal at all. Traditional platforms have a much better failure mode (they hardly ever break) and can almost always be used. Toe clips will aid efficiency. Trail shoes/boots are plenty stiff enough for expedition cycling.
The way a bike fails is important for expeditions. You want "graceful degredation" where you can still ride the thing even when broken. Most modern technology works reliably and well but has a poor failure mode eg look for gear shifters with simple actions not fancy trigger switches.

How hilly is the terrain, most of Africa is fairly flat or rolling but there are mountains in the Rift Valley area.