Wow, your tank is going to look amazing once it is finished and even better once the plants have filled in. I' m looking forward to seeing more of this project.
Btw, are you buying juvenile or semi-adult discus, cause the first thing that comes to mind is, how easy is it gonna be for waterchanges every other day in a 125gal? If you go with juveniles.
Are you planning on filling in the little holes between the cork bark with some smaller pieces of cork cut to fit the size?
The lights just make it look 2 times better! I would go with the 2 filters, for even distribution of filtration, the in line heater and CO2 reactors, and if one fails, the tank won't be totally without filtration.