This is a tough question for the fix-it forum. We really don't have quite enough information.
Frostlines? Heated building? Steel piers or concrete?
Frankly, I would recommend getting the opinion of an engineer. Your city or town might have that person on staff, and they may be willing to provide unbiased opinion. The problem with the 6 feet of fill is we have no idea how far down it is to a good solid foundation, 6 feet of fill above 3 feet of loam above 3 feet of sand above 2 feet of clay and finally hard gravel is a 14 foot pier (less the frostlines or slab thickness for piers at slab. You also haven't defined the frostlines for us. Putting a building over fill does require piers, and steel piers in Minnesota would typically cost 10-20k for a job this size. My knowledge on cost is limited to a couple of houses done next door that required piers.
If you want to do everything yourself it won't be impossible, if you want to do the building right, you really should consult with someone without eyes on your pocketbook for the construction work. It will be well worth paying a structural engineer for their advice if you can't find help from your local authority (and they might not be allowed to help).
If you get a structural engineer, he/she would also do a layout for the concrete piers and rebar in the floor that would reduce/eliminate the likelihood of your floor settling when the fill does over the next 20 years.