As configured it expects there to be a separate wifi router access point that it links to and the clients also use to then access the esp8266 print functionality. If you want to do it without a router then it would be fairly straightforward to change it so the esp8266 ran the AP itself and the clients would then connect their wifi direct to the esp8266.
Also the software is set up to support IPP (Internet printing protocol). This should make it easy to print from tablets, phones etc.
The printer does come with different interface option cards. These look like they are converting from standard interfaces (centronics, serial, usb) to some internal interface which is probably some form of parallel interface. The centronics one is do-able as the link shows, but if you had access to alternate plug in cards that supported a serial connection, then that might make it a bit easier from a hardware viewpoint.