CV 35 for low light
I have and use the cv 35/2.5 for low light work, and it's doable, but could of course use the extra stop. It's a matter of tradeoffs: size/weight/price and which other lenses you'll eventually want. The great thing about the 2.5 is its size and weight: you'll never hesitate to bring the camera along, which is a big plus for me, since that was why I wanted an rf. Others who own the ultron will chime in re size and weight, and it may be the difference isn't that important. I've also heard that the ultron is lower contrast, so take that into account.
Good light and good luck,