I'll answer your first two questions:
Technically, the road can be done with a normal car, but practically it can't. Too many bad patches, particularly between southern Egypt and northern Kenya or Uganda.
That same stretch is also the most insecure. People are kidnapped, robbed and murdered on that stretch, though plenty, particularly locals, do travel the distance.
To my knowledge, there is no passable road from Aswan, in Egypt, to wadi Haifa, in Sudan, which means you have to travel along the coast of Sudan up to Port Sudan, west to Khartoum and then on to Juba, from where you can get to Uganda. This route will allow you to steer clear of Somalia and northern Kenya, which is the most volatile area in this region. Well, as long as Sudan and South Sudan don't start bombing each other again.
From Uganda south, the route is passable with a regular car without a problem.
But, still, you will struggle getting visas for Sudan and South Sudan. And (November 2012) I've been told by several sources that Khartoum to Juba (or vice versa) could take one to two weeks due to lack of scheduled public transport.