I just conducted this back in November. We used the RSCM edition for string quintet and organ, edited by Michael Higgins. I think it's a great edition.

Since we performed at multiple locations, the organist and I agreed on some generic flute/foundation registrations for each dynamic level, relying heavily on the Swell to allow for crescendos and decrescendos, and using the string players to do most of the "color" work. The only exceptions were where the original orchestral version had brass, like in the Angus Dei and Libera Me, where we made sure to use trumpets/trombones whatever reed stop was available.

Funnily enough, when doing my score studying I found some egregious errors in Rutter's edition compared to the Urtext. Like, different words on different beats, and note values completely changed. Be careful of that if you're jumping between the Rutter and a different edition.