As other Peugeot owners will testify, it can certainly be a worthy upgrade, YES. Limited myself to this or that, I usually focus on one part(s) or category of parts at a time. As crucial and consequential as wheels can be, the financial effect can be equally as imposing or impactful. Despite what some LBS workers might say, a used rim or wheel can provide fine service for a season or multiple seasons, assuming it is re-trued and the bearings, replaced and re-packed.

Make sure your inner wire is stainless and not galvanized. You have a freewheel, right? If you have a cassette in back, yeah, you may need to replace it... if it is SunRace, absolutely replace it. The rear derailleur doesn't require replacement on a regular basis so if you're just upgrading to get better performance/results, don't expect too much from a derailleur change (presumably you're replacing one with something lighter, or faster).