As far as headunits go, you don't have to spend a fortune for one, although do bear in mind that if your stereo is crap then so will your sound no matter how good your speakers are. That being said, a killer head unit won't make crap speakers sound good either.
Take your door panels off and take a look at your speakers. It may surprise you what you have. Turns out, my Ranger has pioneers in it and I didn't find out until recently. If you happen to have the same luck that will save you a good bit of money.
I would buy a head unit first, as well as the install supplies. I got my hands on a kenwood head unit for 90 dollars at Best Buy and it works fantastic for the money. A more expensive setup would ultimately be better but for a budget it's great bang for the buck.
My particular model has several equalizer settings, from a more basic standpoint to a 'pro' type equalizer, allowing you to fine tune each frequency from low to high. It also has a bass booster function to really vibrate your seats.
This is what I bought, specifically. Highly recommended.