300 is a great focal length for me on 645. I do not have this lens. I have only borrowed it from a friend. I don't think it was "ULD" ("ultra low dispersion" glass), but I was very happy. I did hand hold it a few times, just for kicks. I'd say it is possible if you can swing fast enough shutter speeds, but of course you will get better results across the board with a tripod. I shot it at '500 and '1000 only, f/8 and f/5.6, and got sharp enough pix for myself. I am sure it is easier to hand hold than the 200mm f/2.8, even though it is a longer focal length.
The 200mm f/4 is excellent and dirt cheap, and can definitely be hand held easily.