IF you go with FL rock then i would say tim's 110lbs is a good amount. If you go with fiji rock I suspect you might have a hard time putting 110lbs of rock in a 55 and still have a fair amount of room for the fish to swim and move about.
This is because fiji rock is less dense so you get more surface area per lb from fiji rock than from the FL aquaculture rock (on average).
As far as the sand shoot for a 3-4" depth if you want a DSB
. You can use our sand depth calculator to estimate the amount of sand you will need for that depth. Remember the calculator is only an estimate because different types of sand will weight different amounts per lb depending on if its dry or we or how large the particle sizes are.