Archie,
I understand your thinking completely.
Did you consider offering two versions? One for what you have called the hobbyist market, the other as a viable sellable product with such capabilities as OPC. As you are aware, if you are an OPC client, then you won't have to expend time creating drivers in that they are already created through the server product for KepserverEX.
On the otherhand, going that route, you would be competing with many larger companies with deeper pockets.
Client Ace is a .net toolkit for creating an Application Programming Interface (API) and provides users of languages such as C# and Visual Basic .NET with a simple, intuitive and optimized class library to quickly develop OPC client applications for accessing OPC servers.
I have not yet taken a look at your HMI to see how it compares with others. If it is as good as advertised, then you could have a larger market, if you expand the capabilities and invest in the product.
Not being familiar with your story either, I don't know if this is a career path you are in the process of creating or just a hobby.
I guess the question to ask is...
Is there a bigger market for your product if you expand its capabilities?
Shalom and God Bless,