I’ve gone through your all posts in this thread. Your effort is really awesome. We also tried designing hardware from scratch. The cost became too high while we provide the sufficient memory (4+GB) for transaction logs, user database, multiple arduino chips for parallel processing, and ethernet/gprs/wifi processing, handling offline data and management online communication adding additional runtime memory and etc.,
At the end, we worked with the existing hardware vendors and we made the biometrics device adding with the motherboard we made. Now our machines can be used for integrating with ErpNext without any support of desktop application. Machine supports rfid, fingerprint, face and palm attendance types and the no. of users supported in the devices varies from 500 to 100,000. Refer [WORKING] Attendance with Biometric Integration through API for messages from our satisfied customers and refer http://camsunit.com/product/home.html for our product list.