You're trying to pack years of experience and hundreds of hours of study into one question. I'll answer what I can. Your only rational choice for a company of less than 75 users is Windows Small Business Server 2003, R2. That gives you your domain, web hosting, Exchange for e-mail, SQL for databases, Sharepoint for file sharing, Remote Web Workplace, Outlook Web Access, ISA, etc. It handles routing and remote acces, VPN's, mobile access... Everything you need in one package, for a very affordable price. Since it is considerably wizard-driven, it's easier for a novice to configure (a bit harder for us system engineers, who are used to doing it all "manually"). Adding users, computers, etc. is as easy as "click click click".
Don't even try Linux. It's free for a reason... hard core computer buffs love to play with it, but for a novice? You'd spend a year just figuring out how to use it, add the features you need, integrate, etc. Not a rational choice.
One SBS server, properly designed and maintained, can run your entire network. SBS 2003 is extremely stable. With hundreds of clients for the past several years, we haven't had a single true "software crash" yet.
Whether you add other servers or not is up to you. You design your network around your business, not your business around the network. SBS supports member servers, stand-alone servers, even Linux and Unix servers can participate in the domain if you get that irrational.
The default group policies and file security settings are fine for startup, and easy to modify as you identify your needs. Ultra secure policies such as IPSec are supported, if you're truly paranoid. You can manage access to files easily, allowing individual or group access to various shares, with various levels of access, all on the same server. No need for seperate boxes.
DFS is really best suited for large environments, complex routing schemes, or with multiple offices. The advantage is that identical copies of your files are stored in more than one location at the same time. When users access a file, DFS draws it from the most convenient location. Also, if one file server goes down, the files are stored on others... making for zero downtime while you repair the downed server. The only real disadvantage is that, with the same files stored in multiple locations, you're going to use up a lot of hard drive space. But, even huge SATA drives are pretty cheap.
Add Symantec Enterprise antivirus and a decent firewall, configure the settings rationally, and you'll have a first-class, clean secure network that virtually runs itself.