Bayo on laterite (filling sand), what size of truck are you buying at 20k?
I use 11 trips of Mack truck load of laterite to sandfill a foundation quite larger than yours.
I also use 1 and half loads of 30 tons granite and 2 loads of 10 tons sharp sand to cast 4in thick slab on 2 units of 2 bedroom bungalow.
On cement, you can’t use more than 50 bags for the casting of the biggest building. You can do more if you have the fund.