Hi. This should be a pretty simple project. The main issue will be low power consumption for long battery life. That's easily solved with careful hardware and firmware design so the device is in a low-power sleep mode most of the time.
I suggest using an off-the-shelf controller module at least for the initial version, to reduce development cost. These are compact, under $20 each, and include a USB connector, so all we'd need is to hook up a battery and the keypad, and develop the custom firmware.
You might also want something (LED, buzzer) for user feedback when entering codes; also the firmware probably should have an encryption feature to prevent user hacking.
My bid is to document the hardware and software design, deliver two functional demo units, and provide well-documented, cleanly-coded and efficient firmware source code. Also PC software for configuring the device and reading out stored data.
Thanks for considering my bid.