ebay has that box for $799, so yes you will be looking at something over your budget. only think I can recomend is to ride out the system you have now until you can afford a better one. I know that you want it NOW, but don't settle for second best. You would hate to know what I paid for my system and only the system. Not even going to talk about the install.
And possible one amp if you can run the subs in parallel
EDIT... Just read the instalation guide. At the end it recomends that you use a one channel amp. advising that, you are good with one amp that is 5 channels