Well, generally they say buy the newest/lowest mile one you can find/afford. There were changes in the C5's throughout the run. The early versions (97-00 IIRC) use a different BCM which are no longer available (as I understand). This goes back quite a few years and was, in part, the reason I bought my 2001.
Remember, the newest C5 is 12 years old and as they age, things become harder find if they break.
My 01 was an automatic coupe that we loved and the man I sold it to loves it as well. I bought it with 66K on the clock and sold it 5 years later with almost 100K. The last year we owned it we took it on a two week, 4400 mile trip to California and back. It was a great road car and we had plenty of room for luggage. We averaged 27mpg for the entire trip. Not too bad for a car capable of 170mph. Most of the trip was driven on interstate highways and at speeds of 75-85 for most of those miles.
I would encourage you to look at the later model years, but if you find an earlier year that you like, don't completely discount it.