Mossberg 930 SPX is a decent choice for an auto gun on that budget, and have money left over for ammo/accessories. But the mentioned FN SLP is probably even better, if it's found within the budget - excellent choice, IMO.
http://www.google.com/search?q=fn+s...4QrQQwAA&biw=1659&bih=842&fp=e18ec2db23988f7d
http://www.google.com/search?q=moss...n&tab=wf&biw=1659&bih=842&fp=e18ec2db23988f7d
Practice like you play. Forget the 24", IMO. Only reason for those is gamesmanship, to extend the mag tube. Unless you're REALLY gonna use the 24" for home defense (which is terrible for going around corners), then forget it and run with an 18-19" with a full length mag. Practice like you play (i.e. fight).
http://www.youtube.com/watch?v=5hfYJsQAhl0
Nice!!!!!!!
FWIW: I'd listen to the advice of the guys (Olyeller, Mike Pinto, Kurt Miller, Jesse T, a few others) that have actually done this.