How hard it is to do depends on your understanding of electronic design and your determination to do it right. Something like this could be done on vero-board (stripboard), which is easier, with similar results to one built using a custom fabricated PCB, the difference will be small but in favour of the the PCB. To actually design one is not hard. For the most part the work has already been done by the component manufacturers, all we need to do is take their suggested application and tweek it for our desired environment.
Here is a simply balanced line driver/reciever unit
Using this as a foundation we can make a unit to whatever quality spec you want. All we need to do is add the main mixer and line level driver (unbalanced) and design a powersupply and case.