I have an old 900GHz computer running as my mailserver at the moment and I like to replace the system with my dockstar. At the moment, I am using a combination of:
- fetchmail
- exim
- spamd
- imapd (ancient version)

I would like to replace the imapd with dovecot, but I am not sure if I should replace exim and which alternatives are possible.

If you like exim, there's probably no reason to switch. I can't imagine it'd use that much more memory than some of the alternatives. Personally, I prefer postfix only for its configuration; I never noticed its memory usage, but with a swap file I don't worry about it that much as I don't get a lot of mail sent to my Dockstar (and it's all spam that I simply forward to another system, anyway).

If you want to use your Dockstar only as a mail server, you should have plenty of power for it. My Dockstar is running mail, web, ssh, and several other services, and after being booted up for a week it's only utilizing 91MB of my swap file. :-) Just make sure to set up a swap file or partition when you configure your system.