As for the ownership of ads I don’t see how that is any different than ads in print or television. Normally it is the  company who commissioned the ad in the first place not the medium it was displayed on. If you have to show the web site with the ad then I assume you have to get their permission as well.

You were looking for a way to display ads. I suggest you look at this web site http://www.archive.org/web/web.php. This is the home of the Internet Archive Wayback Machine. They have been capturing full digital web sites since almost the beginning of the Internet. I can see sites I created in 1998 when I search. This technology could be useful for your efforts.