No. I love mac server and have used it for years and it's one of the best UNIX servers for small businesses - but for all the details you ask in your question, it's not the right choice for the functionality you mention. The mac you have is plenty powerful but will you really take down your server to boot into windows?

Server running as your everyday mac won't help you be a better developer. Server won't help with image and video editing (and may slow them down) Server is overkill for a small company's (less than 5 to 10 people's) needs for calendaring, wiki, web and email - especially one with no IT background. A cisco small office VPN router costs less than $150 if you don't want to run the equivalent VPN software for free on the non-server Mac OS.

Get server and use it because you want to learn by doing, not by making up marginal reasons to justify it. (I don't think you're doing that but wanted to make the point that running server takes time and expertise)

Server is designed and tuned for multi user performance and not single user workload. Some consumer programs don't run as well in Server (or are not supported on server), but the vast majority will work fine even if you can't call up Microsoft or Apple for support once they hear you are running on a server OS. Servers are designed and tuned to run for a long time and you don't want to be rebooting them to take away the services they provide. You can't optimize for both a client workload and a server workload so there is no free lunch and one or both will suffer if combined.

If you only want to play and learn the answer is YES - run server on your desktop, but your question includes you providing real services for a running business. Only you can decide which tradeoff is most important for your situation, but there are a few to be aware of before going down this path.

Running a cheap server (or even look into vmware to virtualize your server os on a larger iMac or MacPro to isolate the server OS from your desktop OS). In practice, you won't likely be slowed down with a distinct (or virtualized) server on the local network (even slow 100Mb ethernet). Your wiki and VPN will be blazing fast with only a handful of users connecting.

It's really hard for even a workgroup of 25 people to bog down a current mac mini server. They really don't need fast CPU or disks. Adding low end RAID storage and FireWire 800 allows the mini to scale up. It takes a special video workload or hundreds of users to need Mac Pro Server. You can learn most of what you need on OS X client by starting up the server processes on the client and save yourself the $499 license fee. I would wait since Lion is announced to have server included at no extra cost.