I'm looking at a few different options with Nathan right now, one is a similar design to yours, Bash500 amp, TCS 12" driver, ported 24*24*14. Another is a sealed 12" with dual passive radiators and the same Bash500 amp.
I'm thinking if you can build a 13Ultra killer for $1200, you should be able to build a very good sub for $750-800.
Sure you can spend that budget alone on an amp, but the EP4000 is only about $350. The DIY guys should be able to recommend something good for a bit less, same thing with the driver.