If you like the color from your Mamiya vs. the color from your Pentax I would suggest you take a look at Fuji lenses.
The only "normal" (135-150mm) lenses that are wider than f/5.6 that are worth anything from your viewpoint are the Planar/Xenotars or the Xenar/Tessars. Unfortunately though, all of these lenses are "older" except for a few Planars that were multicoated. Those late Planars are waaaay out of your budget, and almost anyone's budget for that matter. You might be happy with the Xenotar 3.5 mentioned above. I am not sure if any of the Tessar/Xenar f/4.5s were ever multicoated. Maybe someone will chime in.
You might have to decide whether the shallow depth of field or the clinical quality of the color is more important to you.
I haven't seen many modern longer lenses that were wider than f/5.6. A friend of mine has a 300mm f4.5 Ilex (Tessar) in a #5 shutter that is coated. There may be other Tessar types out there, maybe even something multicoated, but I don't know. I think there was a Docter 210/4.5 which would be modern/multicoated I believe. In getting a fast longer lens you will pay a huge weight/size penalty. Even the 5.6 plasmats are starting to get huge in the 300mm focal length. You may find a 240mm length to be the better bet.
On the upside, whatever you buy, if you don't like it you will be able to turn around and sell it for the same amount of money, or even more. Prices seem to be going up slowly these days.
I hope that helps.