How to manage similar Projects in different ERP solutions?

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project-management

We are working on a ERP System for our company that has a quite number of small projects like vehicle management, customer management and so on...

Now other companies would like to buy some of our small projects (for example: vehicle management). In that case I would like ta make a new solution (new ERP application) and just share this small projects beetwen them. In that case every company can have custom functions but those witch are used in more than one, and are actualy the same, can be shared.

We already have DLL's for basic functions to manage SQL data manipulation and presentation. The ERP solution ist just a collection of WinForms and Reports that uses the DLL's.

As I know there are 2 possible ways to accomplishe that:
1 - Make the small Projects also as DLL-s
2 - Make the small Projects as Single Solutions and include them in the ERP solutions

By 1 way the Debugging is a nightmare :(
and by the 2 way it always throws mea error that the assembly of the small project solution can't be found - even if i add it to the references and using directive :(

Is there a best practice for things like this?