On a Cherokee, you really can't do a quality job by just sanding. Unfortuantely, there are paint shops that do just that. At least on my Cherokee 180, there were many standard rivets with oval heads (i.e. not flush). There is no way you can remove all the old paint around these rivets with out a striper. The risk with a sander is that you are most likely will sand some of the oval head off the rivet.

If the current paint is failing, I wouldn't paint over it. I would strip down to the aluminum. Like others said, you can save quite a bit of money by doing this yourself. The problem you'll need to think about is taking off the control surfaces. You'll need to remove them to remove all the paint. You can take them off, but you'll need an A&P to sign off getting them put back on.