Here are some more thoughts coming from a former user of a P45+ for almost 4 years.
I traded mine in for a IQ160. Looked at the P65+ for less but just knew I had to have the LCD since I wanted to move to a tech camera.
I had one of the P45+'s that didn't go to 1 hour. It had to be physically upgraded by Phase One, which was done under my value add warranty. Note to buyers, since all P45+'s are used now and most are out of warranty, make sure the back you are purchasing will take the latest firmware. There were quite a few that would not and you have to have the firmware to get to 1 hour. This is one reason to stay with a dealer on a used purchase as they can test this for you.
As for the 1 hour:
Well, I wanted it for night work, as my longest day shots are around 30 seconds and I can easily get there with my 160 at 50 iso. Working with the P45+ at night was much more limiting than I originally thought. Mainly since I was not carrying a laptop, and could not shoot tethered. If you shoot for 1 hour the back has to be able to stay on for an additional hour for the necessary dark frame. You will get about 2.5 hours on a normal phase battery. The P45+ can take the canon external charger however so in theory you can go longer.
File are clean, amazingly clean at 50 iso, anything higher and you start getting noise.
Temperature, many this will be a big surprise, you are only going to get a clean 1 hours shot in temperatures under about 70 degrees F. If you go longer in higher temps the back will get too hot and you pick up a red cast with a lot of noise. I used to have the actual chart from Phase one, but I can't find anymore. Living in Arkansas, and the Southwest, this eliminated me from using the P45+ for almost 7 months of the year since our night temps will usually stay warmer than the 70 degrees F. It's great in fall and winter, but that was about it. You can still pull a good 30 minute exposure up to around 80 degrees F. Humidity also plays a big issue and Arkansas, has plenty. This issue of temperature never gets brought up, so I did want to mention it.
Stack vs single long frame, I long ago switched to stacking for my night work. I work with the moon and prefer to have a 1/2 to 3/4 moon on the nights I work. This gives the sky a much more pleasing color, usually a shade of blue. If you work with no moon, you always pick up a yellow cast to the sky due to the light pollution. This is harder than you think to fix as the stars or trails will also be altered when you try to work on the sky. Stacking give you a much much larger range of stars. You can read plenty about this on the web. It's a much more involved workflow but in the long long pays off. IF you stack with a P45+, the dark frame will get in the way and create gapping. You can get around it with software but it adds more to the workflow. You can't turn the noise exposure noise reduction off. Plus there is no remote I know of sold that has a inter voltmeter that will let you set up a series of exposures, i.e. 30" exposures for 40 minutes for the DF body. So when I realized Stacking was where I wanted to go, the DF/P45+ became less of a issue.
Wind, clouds, planes, etc. Stacking allows you to have more control over the shooting. You can still work at night even with a breeze as more than likely one of the stacks will have nominal wind pollution and can become the base for the rest. Planes, there are out there and fly every night and can become very hard to get rid of in a singe long frame.
Lenses, you will get the best night view with a lens wide open. So a F2.8 lens is better than an F3.5. Plus most of the wides from Mamiya, 35mm, 45mm and 28mm are terrible wide open. You will end up cropping up so much of the image you might as well have started with a 35mm. You still have issues like CA and even worse Coma. Capture ONe will do a good job on CA, but I have yet to find anything that really takes out coma. You can find a pretty decent 35mm wide angle @ F2.8, it may take a few, but there are more out there. I never could find a good Medium format lens for this. Briefly though about using my Rodenstock 28mm on my P45+, but all the shutter release and the rest, I figured out not worth it.
These are just one person's options, but if you are only thinking about long exposures for a P45+, I would rather have a 5D MKIII or D600. The rest of the abilities of the IQ series make the gap worth it for me. I am still grateful that Phase One had the upgrade program for the P45+ to IQ series other wise I would not have made the jump.
Paul