Honestly, I haven't suggested anything as I'm not sure if it would be perceived of lower value because of the Northeast GBBR and because I don't know what the Real Steel top rail is worth.
As this is unique and looks far more usable than either the original TM FAMAS or Cybergun's updated version with the basic MOSFET, I'd guess between £350 and £400 if you find the right FAMAS fan but I openly admit I could be way off with the top rail.