That'll be perfect for 1 sa-8 as long as signal isn't clipped / gain is set correctly. For 2 I'd definitely get a little more power if you haven't already bought the amp.
If you have the space and want to save money by only buying one sub, you can go buy 1 12 or 15 inch or larger sub. FU car audio and Sound Qubed seem to be the best performace/price ratio right now if you're buying new. Another good option would be searching the classifieds on here for a good used sub.
For box, hit up cscstang. Getting a box built by someone who actually knows what they're doing and has built for plenty of people and gives results is a must