Hi Jen,
I am a KaBOOM! Project Manager, so I thought I would take a crack at answering your questions:
1. The majority of the holes will be 18" in diameter, though some will be 12" in diameter. Now, those are the specs for Playworld Systems' Equipment; I am not 100% sure about other vendors. Most of the holes will be about 24" in depth, with some also reaching 12" and 36". The installer will lay out where the holes go and direct how deep to dig them.
2. On a recent project, Benjamin Franklin Elementary in New Orleans had to remove asphalt. You don't necessary need to remove asphalt in a 50x50...the dimensions should match whatever the actual size of your playground is going to be. Ben Franklin worked with local contractors to remove the asphalt. It's a pretty huge job, and even more, you need to think about how to dispose of the asphalt. I believe it was about 4" thick, but Ben Franklin ended up having some rebar and concrete underneath, as well. The asphalt wasn't nearly as difficult to remove as the concrete. So, I would recommend that you call around to local construction companies to get an idea of costs and scope of the project. You cannot install a playground over asphalt and use EWF as the surfacing. You could, however, look into using rubber tiles or poured in place rubber, which requires a concrete or asphalt sub-base. If you cannot get the site prep donated, you may want to think about putting that money into the rubber surfacing, instead.
3. As for the soil test, you could dig a hole in a corner and then partition that area off for the kids. I would not fill it back with concrete, because that's going to be tough to remove later. I could put her you in touch with the principal at Ben Franklin, since they had pretty much the same site prep issues.
Let me know if you need more info! erica@kaboom.org
In play, Erica