What kind of Africans are you getting? Mbuna?
Planted tanks usually don't work well with african cichlids. They love to tear them up
If you were going to try any, I heard that big leafed plants like anubias sometimes make it. This works out well since those are also usually low lighting plants. Standard stock lighting would probably be okay for those but 2-3 watts per gallon would be better. Personally I wouldn't focus on the plants in a African tank.
For 75 gallons and up, alot of people like canister filters. Look up the 'fluval 405'. I haven't ever tried canister filters myself so no help there. You could get away with just using 2 hang-on-back filters though. I would get 2 aquaclear 70's for a 75g. It may sound like overkill but african cichlid tanks are usually overstocked to help reduce aggression, so extra filtration is a must.
Two 150watt heater's should be fine.
And yes, I would get more than 40lbs of substrate in a 75g. I used about 50lbs in my 55g and it worked out well. This is really more for looks anyway so it doesn't matter much. If you're getting mbuna, they usually prefer sand though. I would recommend pool filter sand.
Good luck with your new tank! Do lots of research.