One of the most important compatibility factors to consider with email is the mobility question. How often do your employees need to access email via mobile devices? That's an important issue because most email hosting providers deliver some kind of web client usable as a default inbox. Almost all of these can be accessed via a mobile device, so if your employees don't need to access their emails on the road that much, then such mediocre clients are probably fine.
You can also purchase Secure Sockets Layer certificates (starting at $69.99 per year), the software that's represented by a green padlock in your web browser when you visit, say, a financial institution's website. If you plan to sell products via your website, consider an SSL certificate an essential purchase, as it safeguards the data exchange between a customer and your servers.