One of my questions is, once we purchase the domain, will I have to pay for email addresses thru the domain provider? Or will we be able to only purchase a domain and setup everything else up thru the server?
It's entirely up to you.
If you register a domain, you have almost complete control over that domain, including dictating where mail for that domain should be sent.
If you have your own mail server (e.g. the one included in Mac OS X Server) then, sure, you can run your own email, with however many millions of email addresses you want, for no additional cost.
If you don't want to run your own mail server, though (and there are reasons why you might not want to) then you could outsource the email to some other provider - either your domain registrar, or one of many other companies). There might be costs associated with this. You'll have to balance those costs against the advantages and disadvantages of running your own mail server.
I would suggest that if you're new to servers, mail is right behind DNS in terms of complexity of setup and maintenance. For small organizations, outsourcing email is a simple and smart choice.
Thanks, Camelot, for your reply. What would be some of the disadvantages of running my own mail server? I do work in IT so I know I'm capable of setting everything up because I'm a fast-paced learner. It's just that my direct job responsibilities don't require knowledge of any of this information.
What would be some of the disadvantages of running my own mail server?
Let's start with the basics - you cannot, cannot, cannot host your own mail server without a static public IP address. If you're on any kind of dynamic address from your ISP then it won't work.
You also need valid reverse DNS for your IP address. You ISP should be able to handle this, if you're on good terms (or a business-grade account).
Second is spam. You WILL get a ton of spam. Something like 80% of all email nowadays is considered spam. You'll be 100% responsible for dealing with it, as well as absorbing the cost of the bandwidth to get it into your server (bandwidth that might be better used serving real content such as your web site).
Most managed email services include comprehensive spam filtering - and do a much better job too, since they see a greater volume of email, making it easier to spot the spam.
Third is reliability. Users nowadays expect email to be there all the time, if not more. You can't do that with one server - even if you just bounce the server once whenever there's a Software Update from Apple you're still taking your mail offline and someone will notice.
And that's not to mention what you do when there's unexpected downtime - the server crashes, the power supply fails, etc. Oh, and your hard drive just failed? I hope you've got a good backup, or at least no one minds that you just lost all the mail that came in between the latest backup and now.
Most managed email services run a highly redundant service across many (hundreds of) servers, so one server dropping out doesn't affect availability.
Those are probably the big ones. Don't get me wrong, though, there are plusses about running your own, but they mostly come down to control. If you're just starting out in business you'll have way more things to worry about and focus on than your mail server. Get the rest of the business straight first, and then look at bringing it in-house once the dust has settled.