Moving the lift just for storage sake wouldn't be a big deal. Changing location for use could bring about issues. 6 sack mix would be my preference, 5 sack is what gets put down in most GP areas. 5 sack is 2000 psi compressive strength, 6 sack is normally 3000 psi. Just putting foundations under the corner posts is a fine idea, just make them large enough so you have room to miss them by a few inches.