Does it matter which Output Color Space I choose when saving the DNG files?
Starting with the biggest color space available is a good idea. You’d also not want to change color spaces too often.
What options can you choose from?
I tested with a Gray Color space and there is no change on the resulting DNG.
So, it seems Output Color Space plays no part on the conversion.
NLP will happily convert whatever you throw at it, no matter the space you use. Differences can still exist and come up when the image is processed further, specially when an image contains saturated colors.
If you scan b&w negatives, color space might be a lesser concern, although I found at least one image editor that cannot open images scanned with a greyscale profile.