The cat back parts are sold as a kit complete with all the hangers, mounts, and clamps. There is noting wrong with replacing just the rear muffler and tail pipe, but if in a year or two you have to do the front part it will cost you more because you are buying the parts separately instead of in a kit.

I don't see that you mentioned what kind of car you have, but roughly speaking for a 940, the kit is about $150 and getting the same parts separately is around $200 - $225. The other advantage of doing the kit is that it may save labor in the long run.

Charley