Yep- the POP connector is a good thing, if you're a small office that doesn't want to host your own mail. You set up a profile for each mailbox you need to check, setting time check intervals, etc., and point that profile to the corresponding Exchange user mailbox. The server goes out at the set intervals and checks for messages, and if there are any, sends them into the user's mailbox. Provides centralized storage of the user's mail within the Exchange store, so it is available at any PC the user wishes to log onto.
As far as Roaming profiles, do they really need access to all their user "baggage" (favorites, cookies, desktop settings, etc.) or just their Home directory (my documnets). You can redirect home directory much less painfully. You'll need to spend some time "tweaking" their profiles if you use RP's, to prevent bloat. Reduce temp internet files to a minimum; limit my docs folder sizes; etc. If you don't, before you know it they'll have 10gb of garbage in their profile that they need to sync up everytim they log on. Think long and hard about this requirement.
As far as what to buy, it all depends on your likelyhood of growth and expansion. How much data? Any server based apps need to run? Any more users or office growth in the future? These all determine how much of a server you need. If you're running Exchange, I would only recommend SCSI drives, and for redundancy with RAID. Exch is disk intensive, and any implementation I've ever seen on any form of IDE disk or SATA is pathetic.