I have never built or re-built an arena floor but I do know that exacavating has to be done, and done correctly, for the top (and last) layer to be efficient and safe.
The top layer is generally sand of a certain grade because sand is the cheapest.
One also has to pay strict attention to the depth of the sand so horses doing constant work in the arena do not develop tendon problems. <---too deep of a sand footing, where the horse was boarded, cost an acquaintance of mine a vet bill.
I believe 4" is the maximum depth for sand; I made stand corrected on that.
Hopefully someone with experience in building an arena will come in.
In the meantime, this link may be helpful since you are able to build the barn, I know you will get a handle on the rough schematics provided Arenas and Footing
Also, allow $$$$ for some sort of multi-tasking vehicle and a drag that attaches to said vehicle, so the arena flooring can stay well-maintained and safe.
4-wheelers, sub-compact tractors, big lawn tractors, the farm tractor will work if the arena is big enough:)
Smoochies on the snoot to you for doing this with, and for, your wife