Man, you just can not go wrong with a 500 C/M, I have one and a 501C/M and love the whole darn system. I did find a little vignetting with my 180 and longer with my 500 C/M but not a deal breaker.
I agree with CF lenses as a first choice, best price to performance point really. I just had a 120mm 5.6 S Planar seviced and ouuuch it was expensive to get done but it will be good for years.
Have fun with it, best camera system ever made in my experience!