Hi, it´s a pretty easy thing to build, even if you don´t have much diy experience. All you need is a Core Module (here), a couple of lcds and some ribbon cable. There really is not much to understand. You could also take a look here at the different display options. 2 2x40 character displays are fine, but there are other options which (in my own opinion) would be a better fit.
On that topic in the Reaper forum you posted is another guy who used 8 small ssd1306 displays (a little more down) which would fit exactly above the faders of the bcf2000.
If you need any help, we have a very nice and very helpful community here.
You don’t connect it to the Behringer. You actually create a new midi controller. And with a program like midi-ox and midi-yoke you create a virtual midi port which merges/splits the 2 controllers into 1 virtual midi port. You just select that virtual midi port in your daw and you’re good to go. Some one did this with a tascam US-2400 some time ago.