Don't overengineer the prototype. Make it simple and efficient as a jumping off platform to lead to further developments down the road. Any open printer project doesn't have to start with the technology the proprietary models have. They just have to be proof-of-concept that it's doable. Once that's proven, further developments can be made down the line. Dot matrix is easy to create and cheap to produce compared to the overengineered systems proprietary models use nowadays and it would work as a stepping stone toward that further development.