You don't need PADI, you need a good instructor. And a lot of time. This is not going to be a fast path.
I don't know of anyplace that offers certification beyond 100m. If you want 500', you learn what it takes to do 100m and teach yourself beyond that.
Prism is good, but not what I would be choosing for anything that deep. Probably a good starter unit. You can get years of good service out of one before you even think of upgrading to something to do your dream dive.
What's at 500' that you want to visit?