While I have installed & config'ed sendmail, the package, I think it is easier to config postfix for the MTA. There is usually some confusion over sendmail. There is sendmail the package & sendmail the program which can be found in any of these Debian packages:
nbsmtp , smail, nullmailer, exim4-daemon-heavy, exim4-daemon-light, masqmail, courier-mta, msmtp-mta , esmtp-run, ssmtp, sendmail-bin, xmail, postfix.
Background on the default MTA debate for Debian can be found here
You do not need to install an imap or pop3 server to get paclink-unix to work. For your initial install I advise against doing this. After you have confirmed that paclink-unix is working with just an MTA, only then would I embark on configuring an imap or pop3 server. Why would you want an imap or pop3 server? One of the ways I use paclink-unix is from my Android Nexus tablet I use the Android K-9 Mail program config'ed to use imap & invoke the wl2k transport programs via a web interface. That code is in the paclink-unix cvs tree under webapp. So the short answer is that you can support more mail clients with an imap/pop3 interface than a movemail interface. Mail clients that support the movemail interface include Thunderbird, mutt & most webmail clients such as neomail