Network Setup

I think the goals and questions seem too broad at this point.

Have they done any demo'ing of OS X Server (10.6) & client, and determined that the feature-set
specifically meets their needs (shared calendaring in particular) ?

What is the company security policy ? Yes VPN is certainly best, will you go with dedicated hardware ? Have they worked over a VPN before for file-sharing and are accustomed to the additional delays (some is unavoidable due to the overhead of a VPN connection) ?

For email, have you settled on a provider for the SSL cert or are you already fully familiar with how to create a self-signed one and distribute it to the clients ?

Is it necessary ? Don't know anything about the client.

Keep in mind that without SSL (and/or, when connecting over external networks without VPN), email traffic in transit is not secured in any way ? Even internally (ie their private network) it wouldn't be secured, which is probably a non-issue for some.

So it may not be strictly necessary but is most certainly advisable.

It doesn't sound to me like you even need a server to start out - making things overly complex and spending time and money getting it working/keeping it working can torpedo a new small company's success from within.

Your situation sounds perfect for Google Apps. You can force SSL for all services, do much document sharing (really depends on your industry how well that would work), and the shared calendaring is excellent. The free Google Apps version is easy to start with, the Premiere offers some additional features for a nominal fee.

You would not want to do backups over VPN unless your files were small/text-based and even then, overnight would work best. External hard drives can work well, but it's important your users know how to use them and how to protect them (i.e. never have laptop and backup drive in same bag!).

Spend the server money now on getting better Macs, redundant backups, and at least two different types of Internet connections (i.e. cable and DSL) so you are never likely to be down and out. You can either have the 2nd connection on reserve, or put the bandwidth together with a multi-WAN router (I use and like Peplink's Balance routers http://peplink.com) for maximum bandwidth use and automatic failover.

Without knowing more about the company/industry, it's hard to say much more, but in most cases, 6 months or a year down the road, your needs may change, and server hardware/software will be cheaper at that time, and you'll have a better handle on your needs.

I agree that you do not want to spend time (a new company's most precious resource) maintaining your own server and email and calendar when inexpensive outside services are available.

What business do you want to be in, the 'maintaining a server' business?

Find the best no muss, no fuss tools and go with that. You can share files at the home office with plain Mac OS X, without the complexity of maintaining Server.