here is my opinion on the matter:
1) I would take out the crushed coral and go with live sand for a few reasons. The first is that Crushed Coral (CC) will create an area of stagnant water within the substrate that will become a nitrate factory in the future. This is because the decaying matter will settle and slip underneath the surface of the substrate, just decaying away. Secondly, some fish need fine grain sand in order to move around without getting hurt. The top of this list would be sharks and rays (neither which you would have) but this can also include things that perch. Sleeper gobies actually pick up the substrate in their mouths and filter it through their gills and pull out some of the junk, but this cannot be done with CC. Last, I believe that sand looks much better than crushed coral. It just looks more natural, and of course we are trying to recreate the natural ocean.
2) You can buy live sand from this site, or you could get it from your LFS. Either way you would be ok.
3) For a 20g tank I would assume (based on the general rule 1 lb per gallon) you would need 20lbs of sand. You can click on the "live rock live sand" tab to the left and there is a calculator that you put in your tank dimensions and it will give you an estimate on how much sand you will need.
Good luck with your tank, hope everything works out!