That is tricky. You may need more than a mechanical license.
The mechanical license is for limited production. I forget the actual number, but you are limited to something like 1000 copies.
I don’t know if you can limit the number of downloads on iTunes.
A mechanical license doesn’t pay a royalty per item sold. It pays a flat fee to allow you to sell up to a certain number of recordings.
Find out what company handles the royalties for the pieces, probably the Harry Fox agency, and ask them about it. Once you get into actual royalties rather than a mechanical license it gets much more complicated.