I would get something rated for at least 500wrms that way you can be sure you wont be clipping and can get the most out of your sub.

I've built a few boxes for audiobahn subs and I've really liked the sound from their standard ported boxes using 3" and 4" round ports. I've tried slotted ports and they were just as loud, but there's just something about a round port that sounds better to me.