Toyota camry 2009 speaker upgrade

Not really knowledgeable on car audio stuff, so most of that didn't make much sense. What I got from it is that I need a head unit with crossover, which seems to be something that you can plug in an input into and have it output into different frequencies. I'm assuming this is built into the head unit? There are many types of crossovers, does it matter the type in my scenario? Also, for the amp, will any reputable brand with 4 channels suffice? Is it basically a channel per speaker, and 2 channels for the sub? Also, why wouldn't you use rear speakers? Is it because it would interfere with the bass that the sub creates? If I don't use rear speakers, does that mean I will only hear bass in the back of the car and none of the vocals? What would you suggest for speakers in general, as well as subs? I used to have a Honda Accord 2004, and to be honest, the stock was fine for me. If it was a little better (deeper bass), cleaner mids and highs, I would be satisfied. I did have a Sony head unit and messed with the EQ a bit. Taking this into account, would I be better off just replacing my speakers, new head unit, and getting an amp, but no sub? Or should I just replace my speakers, get a new head unit, amp, and small sub? Also, what is hp crossover? Is this high pass? If so, would it just be built into the head unit?