First things first, unless you are strictly mailing yourself or you plan on having discussions with google/microsoft/apple… dump the mail Idea, Trust me it’s not worth it and probably won’t work.
Second for what you want almost everything will be held in ram and the drive won’t matter except boot time, I say almost because you want a samba server so the question quickly becomes how much data do you have and how much money are you willing to spend. I would make the argument that the boot drive should always be ssd but the rest is a matter of patience and money
TLDR: don’t bother with Email, boot is ssd and reset hdd unless you’re rich or not much data.