Hi, sorry for the late reply. Your idea to download on demand is (good drawing, btw) relatively easy to implement
. Whether it is efficient depends on how many disks you have (If you have a lot, it will be more efficient to download a complete / compact database of checksums at once and compare locally while larger images (etc) will be generally more efficient to download on demand).
But the first step is to have good-quality information for download in the first place. Distributing database information to end-users is the easy bit, the real work lies in creating the database in the first place (*) and make it easy to maintain. If more than one person is contributing, guidelines and tools are necessary to manage the database.
I can provide a starting point for such a database by automatically collecting and organizing information from multiple sources, but entries will need to be manually verified / altered as needed. Also, in order to achieve good quality, screenshots and covers would have to be added manually.
(* There exists at least one similar database already (GameBase Amiga), but the licensing terms are much too strict -it does not allow use and/or re-distribution except for with GameBase Amiga)