1. I don't take IR pictures so I don't have any 1st hand knowledge, but I've seen a few report you can get good IR picuters. Here is one such site with sample pictures:
2. If possible always change your lens indoors in a relatively dust free environment and always point the body down while changing the lens. I've heard others say things like change the lens in a zip lock freezer bag ... but that seems a bit much to me. I've got a bit of dust on mine but the cool thing is that it only shows up every now and then. When it does I either clone it out very easily our use Nikon capture to take it out for me with a dust reference shot.
Good luck on your decision. I know I enjoy mine.