Most of the basement in my 1940s house has a concrete floor but there is one small room with a sump pit where it is ~2.5″ of gravel on top of earth (where the earth is ~4″ below the slab level of the rest of the adjacent basement).  My goals for this area are:
(1) Store tools/hardware without accelerated rusting due to high humidity.
(2) Eliminate mildewy smell.
(3) Eliminate other unknown but likely issues due to no vapor or air barrier.
(4) Have a solid surface for the floor.

What is best practice…and what is good enough?

Ideally the floor would be at the same level as the adjacent floor…or lower due to limited headroom.  However raising a couple inches is not out-of-the-question.

I get the impression that less than 2″ of concrete is not advisable.  Is 2.5″ of gravel enough for a non-structural slab?  I guess I am hoping to avoid digging (maybe it’s not as bad as I think?) or raising the floor too much.  We have had a little flooding in the basement once, but that was when it rained for essentially a whole month AND the sump pump was unplugged.  I don’t think the pump has even been tempted to activate at any other time.