It is too early to call it a basket case. Obviously the prior owner did little to keep it up. A lot of archives reading is in order in my opinion.

The black relates to the oil filter I hope. That is normal. It sounds like you will be chasing details for some time. The fuel if black is probably growth in the tank. Maybe even treated and showing up in the fuel filters.

Look up the boost line in the archives as obstruction is common causing no power. Many of your problems may be mostly labour intensive rather than spending money. If your idle is fairly smooth and your transmission shifts well things may not be too bad.

For your leaking prmer pump. Yes that can be an issue. If it is the older design with a white cap rather than black. Stretch a ballon over it and down on the cylinder barrel. Squeeze out the air for now. This should stop any air getting in the system from the primer pump for now until you get a new one.