Hi Peter!
You seem to worry about shutter noise and just about any SLR will have quite a bit of noise because of its mirror. So if you want a quiet camera then you may need to look into a rangefinder or use and SLR with the mirror locked up. You also wanted to know if the Pentax KX and now the Nikon FE whether or not they are professional models. As others have said, there isn't solid definition as to what is a professional camera. But as others have said that both the KX and FE are generally not considered as a pro models although they are suitable for professional use. If you worry about the durability then I can say if you get a good sample of either of them they will last you a long time.