What particular audio upgrades are you wanting to do?
Speakers are subjective so the best bet is to listen to what sounds good to you then get that. There are many good brands out there (as well as expensive) but what matters most is what sounds best to you.
Amps, depends on budget as well as speakers. If you get efficient speakers then you don't really need a high wattage amplifier. However, more power offers more headroom so there's something to consider. For me, I like to have more power on tap if/when I need it (you never know when you might upgrade).
Head Unit, depends on what features you want. Stick to the popular brands (Alpine, Kenwood, Pioneer, JVC, Eclipse, McIntosh, Denon, Clarion, Sony, etc).