Hosted Exchange would be the best solution as it will let you setup the accounts on multiple PC's and they would all be synchronized in Outlook.
I would ask the client if they would benefit from being able to share contacts, calendars, tasks, and public folders, as well as sync with mobile devices. If the answer is yes, Exchange is the the way to go. If they say no, I only want to share the email, then you could go the gmail route or get an app called Osasync. Osasync will let you do basically what Exchange does except it works with your POP profiles in Outlook. It can be buggy. I have only used it once and ultimately ended up switching the client to Hosted Exchange.