In the future, would you rather reflect on your medical school days with a nice vintage, American Trek or a Walmart bike that would probably die on you before you graduate. . Point is, that's a quality bike and putting in a little bit of money and time is more worthwhile in the long-run. Chains are cheap (KMC makes great chains) and the finicky shifting is probably due to your rear derailleur (you might want to tighten your shifters too). To do that only takes about 10 minutes and a screwdriver. Make those changes before thinking about replacing others things like the freewheel etc. More than likely, your bike will run good as new. If you need help, this forum seems to be a friendly and helpful place.

Oh, and nice bike!