Which rim to build a 27.5" rear wheel?

I'm building a new wheel for my son's Whyte mtb - the original hub is shot.
We are using a Hope Hub and am not sure of the best rim to use.
He is using a 2.4" tyre.

These are some we found :
Stans No Tubes Arch MK3 - internal 26mm - weight 426g
DT Swiss EX 471 - internal 25mm - weight 500g
DT Swiss EX 511 - internal 30mm - weight 535g

Lighter is better but what is the best width for his setup?

I noted in a review of the DT Swiss EX 511 that it comes with Squorx Pro Head Aluminium Nipples 2.0mm and PHR washers. Are these worth using?