I have an oldie rb 65 and actually it's very good. That would be the lowest end option and I think it'll satisfy you. Just be sure to hood the thing. As I mentioned elsewhere, the rb 50 is the only rb lens that disappointed me. But then again, I was comparing to the 50 on the mamiya 6, so...

Now there is no denying that the new 50 uld is at the top of the heap, in terms of edge to edge performance. It will depend on your ultimate goals as a wide angle shooter, whether it's worth waiting and saving. I think in the broader context of how much wide angle performance has cost in the past, a used 50 uld at US$1000 or so is actually quite a bargain.

I'll just add, for amusement, that I think the rb fisheye is shockingly good! Good enough that you might consider shooting that and cropping. Especially if you are open to post-scan corrections, I think it is a really under-rated option for ultrawide. Not in the same league as a 50 uld or 65, but I am just saying... well maybe I was expecting it to be complete crap and that's why I was so pleasantly surprised...

I will be playing with the 75 SB this weekend and can comment later.