Do you already know what you want to power with it? Is it to keep a phone charged, or do you want to power other devices?

In my experience solar powered power banks are basically useless (they aren't able to fully recharge their small battery even in a day of full sun); at a minimum you'll want to look into a panel that is at least a quarter of square meter big (they sell a number of these that are foldable and/or have handles for portability).

If I remember correctly, polycrystalline panels are less efficient overall than monocrystalline ones, *but* they work a bit better under non-ideal conditions such as cloudy weather or wrong orientation, which IMHO is a huge plus for a portable solution. I can't really find a reliable source for this, so it's something that probably requires some research.

As joeyh mentioned, solar panel aging is not a big issue, as long as you keep it in mind and overestimate your requirements a bit; batteries however are a different matter and will require changing every few years,.