You can create a Site Column based on "Hyperlink with formatting and constraints for publishing" type (you might need to activate Publishing Infrastructure feature on site collection and site level)..
Than go to List Settings -> Under columns -> Add existing Site Column.. Select the newly created column.. (This column type isn't available in List Columns)
Once you add this column, following is the behavior:
"Test Hyper" is the column name, it has a link "Click to add a new hyperlink", once you click:
It allows Browse option, and you can select any Document from any Library..
Sorry for a bit mess, but you can select document from Library.. Also see Upload button at top left corner, you can Upload the documents to Library from right here..
About validation, you can mark this column as required.. But for /conditional validation, you either need to use SharePoint Designer or InfoPath