First and foremost, if you are getting into MF for the first time, find a dealer, and setup a time to shoot with the gear. MF is much smaller user circle and dealers are critical in the repair and support of the product.
A few thoughts on the P45+, I owned one and shot it for over 3.5 years
The P45+ is a very old CCD design, it's 39MP and really not much good past iso 200 and in most cases iso 200 is push and pretty noisy.
The P45+ is very very unforgiving with highlights, so you will need to expose to the left to keep your highlights from blowing out, and trust me they will blow 100%
Your shadow recovery on the P45+ is limited, as most times areas in shadow are mushy and not very well detailed. This is true unless you totally expose for shadows, and of course blow out your highlights, so bracketing is an important consideration for a lot of outdoor shooting
The P45+ will shoot up to 1 hour, however the ambient temp has to be at 69 degrees F or lower, and when it shoots for 1 hour, it has to take a mandatory dark frame exposure immediately afterwards. In fact all Phase One backs from 1 second out do this, even the IQ250 CMOS.
The LCD on the P45+ is very low resolution and will not offer you much useful feedback in the field in regards to focus checking, as when zooming into 100%, you still can't tell much. And the time it takes to zoom into 100% is considerable, lots of button pushing and moving the screen around.
There is no USB connectivity for the P45+ only firewire as is true will all P+ backs, so in field tethering has to be done with a full sized laptop.
The P45+ chip is a 1:1 crop, so you won't get a 100% view only 90%, most times not that important but can impact wide angle lenses.
Make sure your P45+ has the latest firmware that Phase One released, and that it's certified to get to 1 hour exposures. There are possibly still some P45+ backs out there that can't unless Phase One alters their controller card. I know all about this mine needed this.
If you use a tech camera, then the P45+ will be a bit more forgiving on shifts as it has no micro lenses and no build in readouts. The chip appears as one large chip, not a segmented chip, so no tiling will occur.
If you purchase a P45+, I would purchase it from a dealer and see about a warranty, no P45+ out there should be under any Phase One warranty or Value add, as the Value add's for the P45+ were only 3 years. If you have to send this back in for repair, trust me, you want to be working with a dealer, not Phase One directly.
If you use your D600 for long exposures, from 1 sec to 1 hour image quality will exceed anything you will get from the P45+. The P45+ is good and for it's time was amazing, but when it was current, there were no CMOS cameras at 20MP and none CMOS cameras that were out really could come close to the p45+, but that was 6 years ago, almost 7, and CMOS has come a long way.
It's good that you are interested in MF, but I strongly would recommend not spend the 8 or 9K on the P45+, instead, look at the IQ140 or Credo 40. There have been excellent deals on the IQ140 and Credo 40. This is the same resolution as the P45+, but a Dalsa chip, which is much more forgiving with highlights, much more.
Take some time and contact a dealer and see if you can setup a time to stop by and demo some of this gear, as getting your hands on it will help a lot.
Paul