For 20 gallons of water you generally want 2 pounds of gravel/sand per gallon so you'd want 2 20 pound bags of it. And don't worry about disturbing sand, it will settle right back down after a bit. In fact about every 8 weeks you're supposed to give it a churn anyway so that aerobic gasses don't build up. I just churn a little section of mine every week after I've done a water change so I never forget to do it
And as far as water goes, absolutely no Beneficial Bacteria live in your water column so there's no point in bringing it all over. You just need to acclimate your boy's to the new water when they get in is all. What you do want to bring over is, your plants obviously, filter and heater. Even if the heater isn't on it still carries BB on it. And with the filter keep that running in the new tank for about 2 weeks while the new filter gathers up all the BB it can.
Of course that was me assuming you've got both filter and heater haha, if you don't, don't worry about it. You'll just start the cycle with your plants and the fish.