Using the tag ID is an nice approach and avoids issues with tag formatting and reading.

It's not likely that the tag will be in range and active all the time. They only work at a very short range and are only supposed to be active for a short time - I once accidentally left the phone on top of a tag and it nearly drained the battery in a matter of hours!

It's more realistic to expect that the tag ID will be read, matched and then the signal will be lost. Toggling actions is a very good fit to the expected use of such tags (as per the Sony smart tags) simply because of the way they work...

I got my tags from EBay as I only wanted a few. A good website is www.nfcdog.com and it seems as though my tags came from their EBay shopfront (nfc-supplies). Mine are the MIFARE Classic 1K tags as I wanted a larger capacity to play with. If you're only using the tag IDs then the smallest and cheapest tags you can find will do just fine..