Personally, I would definately go with the Tempest..
As for the amp, you could use the 80 watter, assuming it doesn't mind a 4 ohm load.. BUT, I would highly recommend you upgrade it to the 250 ASAP. You are more likely to destroy your driver by pushing that 80 watts into distortion than if you had an amp with too much power..
Check out
http://www.io.com/~patman/sunosub2.html. It's got a million pictures of every step in the construction of a Tempest sonosub along with a materials list and approximate costs..