Here's how I see it:
Your head unit is 55w x 4 MAX. That usually means 22w RMS, which is worthless.
USUALLY, when speakers are rated in MAX/peak, it is USUALLY double RMS. You seem to have such a huge difference there, I think you have more flexibility than most. USUALLY RMS is a range, and although they may not list a range, you have some flexibility. Even the rating they give is not exact. Your head unit also has 2 preouts, (rear and sub). Going with that, I would be looking for a 4ch amp for your four speakers. Something like:
80w-100w RMS x 4(ch) @ 4 ohms.
Run your four speakers off that.
Before we get into a subwoofer, the basic formula gets us to about 10amp draw for every 100w. A 4 ch amp x 100 is 400w and about 40 amp draw that you need to plan for. MOST vehicles should be able to handle this no problem. Subs use a lot more power than 3 ways and most 6x9s.
Subs can be a couple hundred watts, or more than a couple thousand watts each. When you get ready for a sub, you will most likely need at least the "big 3", a larger H(igh) O(utput) alternator, and possibly an upgraded battery. Figure out your space, your budget, what you have done for your electrical, then get your subs figured out, and then get an amp that will pair with em. Do some research and figure out how it works and come in knowing the basics. People can tell you what to buy all day long. If that's all you want, then go support your local audio shop, and let them sell to you.
You can add a 4ch amp (with power, ground, and speaker wires, and a fuse), and expect a minimum of $200 if you are doing your own work. For your setup, I wouldn't go much more than that. You NEED it to make that power and from there, you are paying more for a cleaner signal/power and a better designed amp. To be honest, I would just look for a deal on an amp that will play at the ratings you need. There's no reason to spend a thousand or even $500.
If you need help with places to search, ask, and I'm sure we can come up with a bunch of websites to check.
