From Nigeria to Chad
 

How complicated is it to travel from Nigeria to Chad. Do I need a visa for Cameroon beforehand, or can I get it at the border? And what about the visa for Chad. Available in Abuja?

AFAIK, you can get the cameroon visa at the border, don't quote me on that though. Chad will almost certainly require a visa beforehand. Personally, the crossing from Nigeria-Niger-Chad and enter via the Lake Chad route through Bol will be more interesting and pleasant (providing you don't get too much hassel from officials or the locals), I guess it depends where in Chad you want to go to and what your mode of transport is.