exchange server is expensive. yes you need a user cal for each user. you need a cal for each system. if something breaks you better hope you can fix it outside companies will charge 100+ an hour to look at it. the server that exchange needs to go on should be a 4quad 64bit system at least..
you need to look at security for it. make sure there is proper mail handling. you dont want to get your ip blacklisted, might want to check into a secondary ip just for your exchange server.
exchange stores email, contact, calendar, notes, tasks, and much more directly on the server, accessible from any computer that logs into the desired account via outlook
unless you archive your personal stuff..
i beleive yahoo might work. i have read that google apps can do similar to what you are asking, you would have to ask probably yahoo tho.
yeah you can copy the .pst files easily from machine to machine, not sure why you want to copy it to the server however..
there is alot more involved. i just can't think of it right this second. i have about a 40 item check list that covers the basic must dos to run your own exchange if you want it.