Is there a common subset of exif metadata tags for scanned photographs?

There's no subset of metadata that is specialized for scanned images, so you'll have to use standard tags, possibly re-purposing some for your own use.

Trying to figure out what tags to use based upon what exiftool can edit isn't the best way to go about it, as exiftool can edit thousands of obscure tags that don't get much everyday usage. You're best bet is to use something like Adobe Bridge (which is free) or a Digital Assets Manager (DAM) such as DigiKam (also free). They provide a graphical interface with all the standard tags, taking care of any differences between the various specs (IPTC IIM, IPTC Core/XMP, EXIF) behind the scenes so you don't need to try figure these things out.

I would suggest creating a text template for the details you want to embed, add a basic description to the top of that for each image, and put the whole thing in the Description. Keep the data in an easily parsable like Info: Details and on separate lines. That way, if you figure out a better solution later on, you'll be able to use exiftool to extract and move the data.