Your best bet would be the HL7 Reference Information Model (RIM) available as part of the HL7 v3 standards.
It makes sense to use the RIM as the starting point since a lot of work has been done to define popular concepts such as patient, encounter, medication, observation...
The RIM is designed to accomodate information elements that are relevant for interoperability purposes. Note the emphasys here on interoperability. As an application vendor you might have more information that you need to store than what is already addressed in the RIM and the various HL7 domains. On the other hand there might be more in the RIM that you need, so you can make your life easier by cutting donw on its size.
Also consider the fact that the RIM is an *information model*, your database design might differ depending on your needs and requirements. Consider disease surveillance and ehr, they both deal with patients (or subjects) and diagnosis, but in a very different way. I would assume that the databases and their designs will be different.
For the reasons that I discussed I have not seen a "reference" implementation of the RIM on SQL Server, but I spoke to several people using a process to generate some of the definitions from simplified versions of the HL7 models.
Don't be discouraged if at first the concept is difficult to grasp, I had (and still have) problems too :-)
HL7 v3 can be intimidating at first, but there is a lot of great work that went into its development and I would definitely have a close look at it before starting from scratch.