Here is a bit more feedback on the wides you listed, and info on the ebay stuff.
Yes, there are a lot of the older Mamiya non digital lenses out there, and sometimes you can find deals on the 150 F 2.8 (as Doug mentioned an excellent lens) and the 75-150 (which to me is as sharp as the 150 2.8 just not as open). You have to make sure you are getting the AF versions of the lenses as there are a ton of the older MF lenses out there also.
The 35mm Mamiya is a lens I used for almost 3 years extensively with my P45+. I had the 28 Mamiya also, but it is not filter friendly and IMO not that sharp until dialed into around F14. I can't speak to the newer LS version of the 28mm, however I tend to feel it's about the same as it has the same groupings of elements. Also Digital Transitions did some very good tests with the 28 against one of the Rodenstock lenses, the 32mm and you can clearly see some differences.
Back to the 35mm, I feel on a 160 class camera, you will not be very happy. This lens is pretty soft until around F11 in the corners and suffers from diffraction softness much past F16. Not a very good range. I believe that Phase One now makes it in a "D" version, but it has the same element groupings, so I would not expect much better. The 35mm is equivalent to around a 24mm lens (35mm camera) so it's pretty wide and it's not too heavy. The AF is pretty solid. It easily handles 2x 77mm filters without vignetting. However it's just not that good in the corners. There is some variance in manufacturing, as some of the versions tend to be better than others, but I tried 3 before I settled on the one I still have in my closet. The 35mm will also tend to smear details in the corners from F 3.5 to around F8. The 28mm I used did this also, but even worse.
The 55mm is one of the sharpest lenses I ever used non-tech camera. It's DOF is very very shallow and you will find yourself sometimes getting into trouble due to this especially on a vertical. Color and contrast are a bit light but the lens when focused correctly will take an amazing shot. I have used the 55mm on a P45+, P65, IQ160, IQ180 and IQ260 and when used within it's focus range it's stellar. You can find them on the web for a very low price point. The newer 55 LS gets great reviews, however since I have never needed the Leaf shutter I have not used it. For sure I would have not concerns about the 55mm Mamiya AF.
Lack of DOF on a wides, and overall lack of sharpness is why I moved to the Acra Rm3di with my IQ160. Image quality is par none, however the tech camera solution does add quite a bit of hassle which so far I can justify due to the image quality. The ability to use tilt on focal lengths of 28mm, 35mm, 40mm and 60mm to me is invaluable.
300mm for me is way to heavy, has no optical stabilization and is equivalent to a 200mm on a 35mm camera, just not enough for me. It's a heavy lens, and for this style of shooting I tend to fall back to 35mm. You can find the older MF versions on the web all the time.
Congratulations on the new camera and back.