I would probably pay $6K for that rifle as long as the bore isn't junk and there isn't anything really wrong with it.
The last couple 461 sporters I have seen sell have gone at around $10K but they were engraved best quality rifles (no Selous plates). They had their small issues but were overall very usable rifles and sold pretty quickly.
I would think that the Selous plates could add value for those of us who appreciate that part of Gibbs lore. I know Ross Seyfreid has suggested that maybe 20 or so Gibbs were made with Selous plates but they still seem to pop up now and then so I would think it could be more like double that number. Still, a very rare feature.