Garf grunge looks pretty good as it comes with bacteria + larger critters, plus I would bet it's pretty biodiverse. A bagged live sand will have the bacteria, but unless you're astonishingly lucky all the larger stuff will have died. I don't know if you need to cure the garf grunge, it seems logical you might need to.
Sand from an established, old tank is best, sand from the lfs will be of variable quality. If it's from there 'super reef display tank' it's likely good, if from the main system it might be good, might be a zero as it's likely been repeatedly nuked by meds so it will likely NOT be biodiverse. Do you know anyone with a reef you can get from sand from.
Remember you will NOT get a lot of the stuff thats so desirable on live rock this way, only some of the bacteria. For the amount of money you'll spend doing this, doing that you could get 5 lbs of good quality live rock and get going.