Level is critical - well less than an inch difference in height around the perimeter of the pool. The soil must be compact - your going to stand about four feet of water on top of it - it will be a problem if the pool water or frame compacts the soil in one area sending the pool out of level.

No need to grow grass under the pool, the pool will kill it.

Block and sand are used for traditional rigid side AG pools, check your instructions but I don't think that Intex calls for them for soft side pools.

I would definitely put a barrier under the pool and the frame. There are several materials available for AG pools but I cut up a couple large tarps for under my old Intex donut.

Remember: Level and compact soil are critical to success. Measure for level, nobody's eyes are that good.