Well, there is quite a bit of what you ask where the answer would often be a lot of opinion.
You would have to decide that if a site that has not been updated for several years, that makes it less credible, simply because it has not been updated. I think of relatively old sites, such as those that might list old MIDI files for download. There would not be a lot of updating needed, as the collection of files doesn't change very much, and the method used for downloading wouldn't change significantly, perhaps for several years. I have seen old collector's sites, giving a source, sometimes the ONLY source, for old light fixtures, or circuit diagrams for old hobby electronic projects, etc.
The fact that a site like that is not updated very often, does not necessarily affect the credibility of a site.
Maybe you have a web design company local to you, where you could ask a few questions. That kind of company should evaluate the effectiveness of a web site, and can offer suggestions about how a site can be improved (with credibility as one criterion for that evaluation). Ask a couple of questions there, and that may jog your ideas about how to do your assignment.
