When I download an item, I would like the modified date to be the date the item was downloaded. Is this possible somehow?
It is possible.
You can change the modified date of a file using this command
where YYYYMMDDhhmm is the date/time you'd like to set it for. Combined with the Add Folder Actions (where you can assign an Applescript that runs when a file is added to a folder), you should be able to create an Applescript to do this (using the touch command above).