If you are considering a used one, make sure it will run the latest version of the P45+ firmware.
This allows two major changes to early P45+ models (like mine was)
1. You can take a long exposure up to 1 hour.
2. You can use the higher ISO range from 400 to 800.
My serial number is 399. Purchased in March 08. Phase released a major firmware update I recall around mid 2009. This update allowed for the longer exposures up to 1 hour and greatly improved the higher ISO range. However certain backs, (mine was one of them) would not take the firmware. There was a change to certain hardware in the backs that worked with the firmware. If you didn't have it, you would lock up during the firmware upgrade.
Phase One, took care of this issue for me. I was under full 3 year value add warranty. The changes were IMO extreme. The new firmware not only allowed me to shoot up to 1 hour with very low noise (iso 50) but also cleaned up the higher ISO range enough that I can use iso 800 for shots up to 24 x 36 with no problems. It also cleaned up the lower iso range 50 and 100.
One other note, one that doesn't get talked about very often.
The use of the P45+ in 30min to 1 hour exposures is very dependent on the outside ambient temps. You can't get a 1 hour exposure in much warmer temps than 63 degrees Fahrenheit. You can go to 30 min up to around 75 degrees as I recall. If it gets any warmer, you will have a mainly dark frame and the exposure will not be usable. Below is a quote from a note I received from Phase One. I had sent them a problem file of 40min.
"This image is a 40min exposure with a core temp of almost 36 C (35.813 to be precise)
At this temp we can handle a 13 min exposure at 17 C you can do 60min and at 23C you
can do 30min so to be able to do 40 he needs to keep the back around 20 C."
Based on their work and recommended temps. I have had excellent long exposure results with the P45+. Outdoor temp can't be over 63 degrees F to get to 1 hour.
Here is an example of approximately a 30min exposure.