A few more thoughts.
The P45+ is able to get to the 1 hour range with the use a custom firmware and a very special controller card. I know this much as I owned a very early P45+ which was shipped in March 2008. At that time Phase was not able to get past 30 minutes (on my back about 15) but promised that with future firmware upgrades the 1 hour range would be met. About 1 year later, April 2009 I started working with Phase One as I had been told that the latest firmware. I want to say 2.8.3 was the main release number would now allow this. However I was also cautioned that certain Phase One backs would not work well with this firmware and lock up, requiring the back to be returned to Phase One. My back was one of these. After installing the firmware, my back locked up and was would do nothing. Phase did fix it and in the process replaced my controller card, reloaded the firmware and the back gained the ability to now shoot up to 1 hours exposures. It also gained a much cleaner ISO 400 and for the first time useable ISO 800. My Kudo's to both Phase One, Doug Peterson (who first told me about this issue) and Digital Transitions my dealer at the time who handled the work with Phase One.
So , be careful when purchasing a used P45+ with a low serial number. Make sure the back will shoot up to 1 hour without excessive noise. My serial number was below 400 but I was told during all of this that Phase was using 2 different controller cards and it was a total crap shoot as to which back got which card, the later card worked mine didn't. I was able to get this all taken care of during my original Value add warranty. This is typical, you job out for a card and have two vendors or more supply it. Certain venders versions of the card would not work with the final required firmware.
Just a couple more thoughts.
I worked with the P45+ for over a year on night work, but quickly soured on it. Mainly because of the dedicated dark frame required. Net shoot for 30 minutes, then wait for another 30 min. while the dark frame was written. Shoot for 1 hour wait for another hour, etc. Phase One's batteries really are good for maybe 3 30 min frames, or 2 45 min, and really only 1 one hour as you won't have enough battery for another exposure. Since I prefer to shoot with moonlight illumination, I have found that stacking works better as you get a better working balance between the stars and foreground illumination. Whereas leaving the camera just open for say 30 minutes tends to wash out most of the stars except for the brightest. This is for Astro Photography uses. If you stack with the P45+, the dark frame disrupts the exposures thus you get gaps. Some of this can be corrected in software but it does add additional work. However once you start putting together a bunch of P45+ frames you realize you need a good bit of processing power. Stacking requires a lot more post work, but so far for me the results have been worth it.
Lenses, since I was using the DF body, I only had the 35mm, and 28m to pick from. Both of these lenses are pretty terrible in the F3.5 to F5.6 (35mm) and F4.5 to F 6.3 (28mm) so I found that really only the center of the image was very good. You are taking about using Hassy glass, so you may see much better results.
If I took either the Mamiya 35mm or 28mm to F 8 or so, there wasn't enough light unless I pushed the ISO to 400 but then noise became a much bigger issue. My 28mm Mamiya was especially corner soft in the Lower Apertures from F4.5 to about F8. My 35 was better but suffered from resolution smearing in the F3.5 to F5.6 range so it's results were not much better. The 55mm's limited DOF took it out of the running.
I have moved to Canon/Sony for night work, just a whole lot easier and much more forgiving. I have a few frames from my P45+ that I was proud of, but most nights conditions just didn't pan out for a single long frame and a stack worked better. With the Nikon D800 now starting to ship, I would consider that also. At 36mp you should still be able to get where you want for a 30" x 30" print or larger. (YMMV), but it's a lot cheaper solution. Nikon will have a great noise reduction program for longer frames and I am sure have the ability to turn off long noise reduction so that you can still shoot in a stack. The results I get from my 5D MKII many times I feel could get to 30 x 30 and I have printed larger but everyone has their own printing requirements. For night work, the 35mm Digital just seems to me a much easier route, has many more options than the P45+ or P30+ and is much less of an investment.
Paul