Building an AV setup- speaker choice

[Herminator]

I'm looking to get a good surround sound setup for movies, tv and gaming mostly.
I have the AV amp, a Sony STR-DN1050.

I'm after some advice on speakers.
My dad has passed on to me a pair of Kef C-Series floor standing speakers, which must be getting close to 30 years old...
Are these worth adding to? ie. buy a good centre, then sub, then probably some new fronts and moving the old Kefs to the rear?

Originally I was thinking of going for a package like the Monitor Audio Mass 5.1 setup, which look pretty smart.
But have been told adding to the old Kefs would be a better idea.

Space isn't an issue. I prefer the looks of full floor standing speakers or the smaller Mass speakers over book shelf style speakers.

Are modern Kefs worth looking at?
Are Sony? (I can some Sony stuff at good prices, which is why I ended up with the amp)

With the room setup I think I'm going to end up having the tv in one corner unfortunately, as the only available wall is in front of a large window, unless I rip out the fireplace...