I did some brainstorming about Number of Digits, Drivers, Microcontroller and Interface. I ended up with the following for the the first main board design:

• 5 Digits wich are connected as 25x7 Matrix (25 cathode columns and 7 anode rows) 
• Driven by:
  • two 16-Bit constant current low side drivers (MBI5026  or similar)
  • one 8-Bit source Driver (probably MIC5891) in the same SPI bus
  • OR one logic demux with p-fets or driver directly controlled by GPIOs
• One low cost 8-Bit MCU (probably some SMT8S) 
  • doing all the LED multiplexing
  • UART and/or I"C as control Interface
• ESP8266 Module 
  • because everything needs wifi
  • Maybe ESP8285 to safe some space

I hop all these ICs will fit on the back of 5 Digits. I will try this the next days.