On the Puch: rarely worth it if you have to put all new components on it. You are better off finding a complete bike with the components you want. Or find a donor bike with the components you prefer, and swap them out between the two bikes, and sell the one with the components you don't like. This assumes you do all the work yourself, as it will cost way too much to do a complete swap on two bikes if you have to pay someone to do it.

If you are desirous of building a bike with certain high end grade of components, you are best off with the best frame you can find (at an acceptable cost of course). It takes just as much time and effort to build up a low end frame as it does to build a high end frame. Note, that some Puch frames are high end, but without pics, I would not assume it is. I would shoot for a higher end Tange frame, Reynolds, or something similar.