Seems like a bit of a waste of an expensive semi pro-level sound card (i.e. AD516, Repulse, Toccata or Delfina) just to hear MP3s on an Amiga (Amiga sound cards are usually quite pricey).
I can see a musician or someone doing video or audio editing needing one of these cards, but if all you're going to do is play MP3s, the MAS player is a lot easier on the wallet and less scarce (you can actually buy one now from AmiKit instead of hunting around on ebay for a used card which will cost at least twice as much). You can actually buy one from Amikit right now, for instance.
As for mixing Paula audio with the MAS player output, a simple "Y" cable should suffice - or just buy the "MAS Player Evolution" model. The MAS Player Evolution model includes a built-in mixer and connectors to accept audio from the Paula output (or any other source):
http://amigakit.leamancomputing.com/catalog/product_info.php?products_id=1078
