I don't work for a contractor directly or indirectly, but I am very familiar with this side of the business in Florida...
What county are you in?
Have you had SPTs drilled on your site?
40-50 ft of muck over limestone would be very surprising, so say my coworkers (geologists). My point is to take a contractor's opinion with a grain of salt, like asking a roofer if you need a new roof...
Helicals can be installed down 50+ ft into limestone no problem, actually, with a lot of clay it can be an issue in penetrating through the clay layers. Depending on your area it has been done for less than $2000/helical, also depending on the depth to limestone. Depending on the model and company, I have seen 20k++ axial allowable capacity.
Mike is right, this is a specialty. But there are areas of Florida where there are a lot of these types of contractors because of residential foundation repairs due to soil issues.
PEinc also has a valid point on the lateral buckling stability of a 3" steel pipe driven down 50ft. We've discussed that in our office before, not a lot of good technical info out there on that. How much lateral support does soil (especially organics) at various depths provide? Unsure, may involve on-site load testing to know for sure.
That's about all I know, HTH!
Andrew Kester, PE
Florida