Why were you looking to upgrade?
You'd probably want to think about what you want the sub to do, then do some research on the different subs. Try the many car audio forums since there's probably a larger pool of experts there that have experience with a variety of sub/enclosures etc. If you are trying to fit a stock enclosure you'll probably restricted to what will fit either by the sub size or depth. Then there's the volume of the stock enclosure.
As for amp, something with enough power to power the sub I guess. And since you have a space restriction, something with a small footprint or runs cool so you can tuck it away.
I'm also doing some research but I don't have an enclosure so I'll end up having to make my own.