Congrats on finding those cameras. I wouldn't bother with the branded flexadapter.
I would simply get a chinese adapter you can find on ebay. The quality is as good as you can expect from those mounts. You will find that the flange focal distance might get in the way of using wider lenses. You can use most analog lenses but will likely find the focal lengths and resolutions to be frustrating or boring (once they have been cropped from a 4x5/8x10 image circle to the teeny phase one sensor. Most opt to find and use digital specific large format lenses (digitar, digaron, sinaron). I mount my Phase One back occasionally on my sinar and horseman cameras. I love the sinar, but the horseman is lighter and because of this, it works better in the field. I guess if you are using h10 and h20 you will be tethering, but do know that this is a fickle system especially if the camera's gearing/movements is/are poor.