Great build Earthswater!!!
As far as pool filter sand goes its usually around $5 for 50 lbs. You can get away with 50 lbs for the 55 gal, but you can always do 100 pounds if you want more sand.
Rinse and stir the sand in 5 gal buckets till the water comes out clear, and then apply to the tank. Try not to dunk it all in there as this will make the water cloudier. Usually by the time all the sand has been applied, wait 30 mins and turn on the filters. Wait another hour and if the sand hasnt settled you can always do a 50 pct water change and it will help tremendously. You dont have any fish in there so i would just leave it with the filters running overnight and that should take care of it!
Good choice on the africans, they are beautiful fish and always active. Provide alot of caves for them with those rocks, and remember to try and get your ph to 8.0-8.2 which is their natural water.
As far as filtration get an aquaclear 110, petsmart has them on sale for $70 so if you can buy one, its a great filter.
Keep us posted!