My advice would be to buy cheap and then trade up if you find that your initial choice doesn't work for you. I.e., start with one of the 115/120/125 wides at ~$400. Or perhaps one of the slower 90's if you can live with less movement and more falloff - those are very common and tend to be inexpensive. So far as I'm concerned the 90 SA-XL is an expensive, somewhat awkward-handling, special-purpose problem-solving lens, not something you want to start with just because the specs look great on paper.
I don't think "invest in the correct tool from the start" tells you what to do, because you don't yet necessarily know what the correct tool is. Your FL preferences for 35 and MF may not be a reliable guide to what will make you happiest for 6x17. 90 gives a comfortable, normal-to-semiwide vertical field of view on 120 rollfilm, but the horizontal field of 90 on 6x17 is very wide. Many users of panoramic formats end up going a bit tighter on the vertical field after they get over the thrill of the ultrawide view and it starts to sink in how difficult it can be to compose with the shorter FLs. By the same token, your assumptions about how much movement you're going to need may or may not hold up once you start working in the field.
My experience with the 90 SA-XL is limited to one test exposure so far, as an ultrawide on whole plate. For that kind of stretch the falloff is very substantial, even stopped down. But I haven't tried the lens at open aperture nor am I likely to anytime soon, I'm afraid. I always use my 115 Grandagon stopped down, too.
I did just take a peek at the LFF thread. My inclination is to agree substantially with Frank.
Now, time to unwind - though truth be told, I'm sufficiently old and creaky that my mind turns more to telomerase these days...