It really depends on what you are going to be shooting. For 4x5 I have always enjoyed wider angle lenses for my landscape work (I use a 75 and 90) and slightly longer lens for portraiture (I use a 250). For longer lenses you need to make sure your camera has enough bellows draw to allow for the given focal length and for shorter lenses enough bellows compression. However, I don't think you'll have a problem with a 250 or a 75 with your chamonix. My complete lens kit for 4x5 is 75, 90, 150, 250. I would have a longer lens, maybe a 300 or 450 if my camera had enough bellows draw.
One thing to consider is if you are going stick with the 4x5 format exclusively, or if at some point you are thinking of moving to 5x7 or 8x10. I set up a nice simple 4x5 settup a couple years ago but now I've moved on to shooting mostly 8x10s. The lenses you choose for 4x5 most likely won't cover 8x10. Each lens has an image circle that is projected onto the ground glass. The image circle for a 4x5 lens is large enough to cover 4x5 film usually with some lens movements. Before you purchase lenses it's good to research and find out some basic information, including image circle. My fujinon W 250 f6.7 that I use for portraiture on my 4x5 also nicely covers 8x10 with some generous movements (on 8x10 it's a moderate wide angle). However my 75 and 90 won't even cover 5x7.
I wouldn't worry too much about the brightness of the ground glass on your chamonix. Especially with a normal or long lens I don't think you will have a problem with light falloff but that's just my personal opinion assuming you pick an f8-5.6 lens. If the ground glass is too dark you can always invest in a fesnel lens (assuming the chamonix does not come with one already installed)
