First off, welcome to HAN. :cheers: Enjoy your stay.
Ultimately, the limitation will still be the stock paper speakers... so if you want quality sound reproduction, the stock speakers have to go at some point.
In fact the higher power of most aftermarket decks have the ability to completely overpower the stock speakers, speeding their demise with terminal distortion.
If you're on a budget, I'd upgrade the speakers first. They may be under-driven by the factory headunit in the interim, but they will do a much better job than the stock speakers could ever hope to do. Then as funding allows, purchase a headunit that matches your system goals.
Moved to Audio & Security.
(It's our catch-all for car audio
)
Once again, welcome. :thumbup: