Lazarus is closer to Delphi than was before, and easy interface design beats the other C group. However, you won't need too much on the interface part to judge on that

If i'm not mistaken you have a scanner there and also the actual server - more speed will help with large collections, but the native languages should run as CGI, so you will lose from request startup time. Something with apache module will fare better for serving. Unless you want to do a whole web server.

Easiest for the user will be a stand-alone installation that will get them all the functionality without messing with config files at all, small IDE for choosing a directory and eventually a tray icon. I think i just described Orb