A few things: what sort of music do you listen to? How much are you willing to spend? How loud do you play your music?

Good speakers almost always need to be driven by an amp to achieve their full potential. Most HUs can output maybe 15 to 24W per channel continuously. You can get good speakers rated at 50W and probably be ok. Huge difference from stock. Hopefully, you will stop at that. But a good 50Wx4 amp won't take up too much trunk space.

Price could be anywhere from 5K to 30K per pair depending on the brand and model. And you need two pairs. Though some people prefer just having a front set.

There are two ways to go: buy the best you can and stick to it 'til it dies, or buy what you can afford right now and upgrade later.