Concerning filters I use myself the Lee Seven5 filter system for ND grads. It's actually designed for compact range finders, but as tech lenses typically have a small filter ring as they are small aperture lenses it works well and is small and compact and you just screw it directly on the lens filter thread. Great when hiking with the gear. I know I'm not the only one that have chosen the Seven5 as filter system for a tech camera, but the larger Lee is more common of course. The quality of the smaller is just as good though. A difference is that the sharp transitions is a bit sharper than on the larger Lee (I've been told), which to me is an advantage.
I have the Seven5 lens hood, but I don't use it nowadays, I shade with my hand or with an umbrella (which I always bring anyway in the case of rain). I think that's a good option and gives me one less thing to carry, except for that I sometimes run out of hands.
The Seven5 will work up to 72mm filter thread which means it fits all digital tech lenses except the following: the Rodenstock Digaron-W 32 (86mm, 105mm with center filter), Rodenstock Digaron-S 23 and 28 (72mm, 95mm with center filter), Schneider Digitar 28 (95 or 112mm depending on filter ring option, and the 115mm center filter if used has no threading at all). My widest lens is the SK35 and it works on that, it does vignette a little but you get as much as 88mm vertically and 80mm horizontally which is not too bad of the 90mm image circle.
You'll also notice that grads (which are always uncoated by the way, even if you get the expensive heavy Schneider glass ones) is hell on ultra-wides especially if already having a center filter. It's extremely reflection-prone. Using bracketing techniques instead of grads is sometimes better when shooting ultrawides. I know some prefer to even not use the center filters and bracket instead for better exposure in the corners, as the center-filters themselves can lower contrast and cause reflections in difficult light.
A good way to start is to figure out which wide angles lenses you are going to use. Even if you don't buy them at once it's nice to have it planned in advance. It's there the big costs and compatibility issues are found. For example you can't use the full image circle of the SK28 on your Credo 40, on the other hand it might still be a better option than the Rodenstock 28 which has a hard limit on 70mm image circle.
Concerning polarizers and regular NDs I use screw-in filters instead. It's a bit more cumbersome and slow to mount but you get a lot more products to choose from and you get it absolutely fool-proof light tight which is good if you would use a really dark ND. It also becomes cheaper. 72mm screw-in polarizer is a lot cheaper than a polarizer for a square filter system. I use the super-thin Kenko Zeto EX polarizer because it loses less light than most other filters, which is good as I have a back limited to 30 second exposure. With wide angles with center filters and shooting at f/11 the shutter speeds are long enough as is.
Concerning cast the Lee grad filters will change the temperature somewhat, it becomes a bit cooler if I remember correctly. I think there's also a little bit sample-to-sample variation. If you need the most neutral you can get I think the Schneider glass filters is the answer. They are designed for pro video use, but there is a kit to buy for still photographers when it comes with a special-made Lee filter holder.