I live in CO and am a do-it-yourself type, I did a flagstone patio in a clay soil and simple put down about 3-4" of sand and compacted with water and a hand-tamper. It worked well and I used soil and grass seed for the filler. It worked perfectly for a patio! I would guess that pea gravel (tiny size.. smaller than 3/4") works just as well for a base in clay. Suggestion: For anything harder than soil or sand, use a mattock for digging (friendly adivce from Colorado).

Oh yeah.. How level? For a patio, try to keep the pieces level with a slight slope from the house, 2 ft level will work fine and stay at about a 1/4 bubble slope. I am not a landscaper, but this worked fine for me.

I am by no means an expert, but I would think the clay doesn't have to be perfectly level, as long as water will drain away from the house. That means putting in a slight slope, with no low areas where the water will collect.

As far as what gravel to use, for my patio I used something which is known in this area as "crusher run" or "road base". This is a mixture of gravel in various sizes and sharp sand. It compacts easily, stays where you put it, and drains well.