[Dovecot] Confused about what MTA, MDA, IMAP/POP server to use

[Netlink Tech]

Hello...I'll TRY make this short. This post is not specific to Dovecot...but many subscribers to this list have probably been down this road.
(private replies welcome if you want it to go off list)
I currently use Sendmail with MailScanner, SpamAssassin, ClamAV,  Procmail, and qpopper for POP3 (hash mail dirs for mbox files)on RedHat 9 server. 
Roaming users currently only have the option of UebiMiau webmail interface to POP3 (doesn't work well)
I will be moving to a server with Fedora Core 5.(x86_64)
Things I would like to accomplish:
1) Move to maildir mailbox format (currently large mbox files cause I/O flushing errors and problems for users retrieving).
2) Use squirrelmail for webmail (IMAP) (local or roaming users)
3) Enable local users (ones with current accounts on the server) to continue to use their current client setup.
4) Enable any user to use the webmail interface to retrieve/send email.
5) Enable roaming users (ones connected from any location) to use Outlook Express or similar with SMTP_Auth and port 587 for message submission and POP3/IMAP.
6) Enable mail quota. 

I want the current users to use their current settings (unless they roam...then require that they use webmail or SMTP_Auth and port 587), but moving forward with new users, I have no reason to give them shell access/system accounts....so I was thinking virtual users using user at domain and storing their auth information in mysql db. (mysql will be used for radius...so it is already running on the server).

Because I am trying to figure out how to keep the existing users system accounts and move forward with new users only having virtual accounts (just mail access with no login), I don't think I can use file system quotas.

I have used Sendmail for 10 years, so I am familiar with it and it has been very stable for me. (so I prefer to stick with it if I can)
I rely heavily on MailScanner to scan/mark/clean virus and Procmail to filter marked spam.
Qpopper is on the way out...A couple of weeks messing with the test server and I already love Dovecot.

Questions:
1) Recommendations please: How to accomplish with what configuration.
2) Quotas?
3) SMTP_Auth using Mysql for virtual users? (works fine with PAM/system passwords)

Sorry for not being Dovecot specific on the list.
Curt
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