[Qwe]

Yea the potting mix, not garden soil: http://www.homedepot.com/p/Miracle-Gro-Organic-Choice-32-qt-Potting-Mix-72983650/100619045

Any gravel I've used has been something I got for free or with used setups... but have picked up pool filter sand at a local pool supply shop. http://www.homedepot.com/p/Quikrete-50-lb-Pool-Filter-Sand-115350/202314677

 

[Qwe]

I think it depends on if you want to sift/mineralize the dirt first, or just put it in straight from the bag. I've only done the latter, which one 50 lb bag would be plenty.

The gravel, 1.5-2" would be about 50 lbs, and I think it's up to you if you want less than that...

 

[Qwe]

It's a process advocated by Tom Barr generally done with top soil, not potting mix. But you basically soak the dirt, let it completely dry out (generally laid out on a tarp or something), then repeat two more times.

Even if you don't bother doing that, but sift the dirt, I'm pretty sure MGOCPM has lots of bark and such in it, so you'd end up with a lot less... but I've never bothered so I'm not sure exactly how much less.

 

[Qwe]

Yea I forgot about that part, just a process I was never interested in... but here's a good step-by-step:
http://www.aquaticplantcentral.com/...-mineralized-soil-substrate-aaron-talbot.html