sftp server for owners/customers

[Al Sparks]

I'm trying to set up an sftp server so that our customers (which I'll
call the owner-accounts) can allow THEIR customers to transfer files
to their accounts.

What is a good way to set this up?

Some ideas I have are:
Two sets of accounts, one an owner-account with read-write to a
download directory, the other, a generic customer account with write
only to the download directory.

I'm not absolutely wedded to OpenSSH's sftp-server, though I guess if
I went with someone else's sftp-server, I'd have to change over all my
ssh stuff for that server (unless I was willing to use a different
port for sftp).  Are there settings I don't know about with
sshd/sftp-server that will allow me to be more granular with CWD and
allow/deny commands much like ProFTPD?

Alternatively, does ProFTPD have some sort of encryption built in?

What other file transfer mechanism with encryption might I use
(https?) that would allow owner-accounts to allow transfers to a
write-only directory?

  === Al