For lower power consumption, I got some NRF51822-04 BLE4.0 modules modules. Bluetooth has the additional advantage that it can be used from a smartphone, so no extra server hardware is required (but optionally possible, with WiFi or ethernet connection). To test everything with the displays, I created a test board:

This is how the board looks like:

You can find all KiCad files and the CubeMX test project for the last test with the Crystalfontz display in the github repository. The board is 10 cm x 10 cm and would cost $76.25 at OSH Park, so I ordered it at seeed studio, for $4.90 for 10 boards. This means each board costs only $0.49, nice price. But for smaller board, OSH Park is less expensive because of the free shipping. The shipping cost at seeed was $12 with the "US GB DE AU Post" shipping option. But a board price of $1.69 is still very good.