I say find a nice strong 4 channel amp and run a set of components off the front channels, sub off of the rear channels bridged, and then if you want rear fill run rear speakers off of the rear channels on the deck (in which case your stock speakers are probably fine for rear fill).

That alone will probably save you some money

So in short:

About 300-400 for a headunit
Probably 200-350 for a nice set of components
And then a nice four channel amp + install

I think you could get it done for quite a bit less than your total there ^^^