That head unit will work fine! It doesn't have a subwoofer output, but never fear...you can use the rear RCA output through a low pass filter and into the amp. Heck, many sub amps have one built in. That's down the road anyway.
Ok, onward now to the speakers. Blaupunkt is that name brand stuff I was talking about. I didn't say you had to spend a lot of money...just that you want to get something from a company you can chase down.
Personally, I wouldn't bother with the OEM Ford speakers. Later model stuff is ok, but anything not built in the last 5 years is going to die a painful death even when only fed the power of a head unit. My factory Toyota headunit put out 4 watts by 4 channels RMS. My aftermarket headunit puts out ~24 watts by 4 channels after some tweaking to the board inside it. That's a big jump!
So...how does one go about wiring what you have? With the speakers like you have an amplifier will be a GOOD idea. Otherwise you will never realize their full potential. I have seen Pyramid amplifiers works rather well, but they keep their specifications rather hazy on purpose. Notice how it says 75x4 total output? That's that struck by lightning figure I was referring to in my earlier post. I would venture to guess your headunit has more power.
I COMPLETELY understand what you mean about not wanting to spend money. Here is my recommendation. Look for a tested, but used 4 channel amp on eBay, craiglist, the local college's free classifieds....whatever! I have had good luck with my used stuff. If they don't offer a DOA guarantee then I wouldn't bother, but deals can be had.
Ok, now for how to actually wire those things. You have two choices. One would be to use the single 4 channel amp. The other would be to use the 4 channel AND the sub amp. I'll start with just using the 4 channel.
After running the power, ground, remote turn on, and RCA wires to the amplifier you should just be left with the 4 channels of output. We are going to load that amp up, but in a good way. Without knowing where you want the most music power I'm going to have to keep this somewhat generic, but you'll get the idea.
When you connect the speakers you will have 1 pair on 1 output of the amplifier loading the section to 4 ohms. You will then have two sets of pairs on the other channel (4 speakers total, two to each output). That will load that other half of the amp down to 2 ohms stereo, a reasonable load by any means. You will have to decide where you want more power. The 4 speakers are going to be louder than the two even though each individual speaker will be receiving slightly less power. Put them in the loud section.
The single pair can just be directly wired to the amplifier. Just match left with left, right with right, positive with positive, and negative with negative. Simple!
The pair 'o pairs will be wired in a similar fashion. Run a set of positive and negative wires from the outputs to the left and right sides respectively. But here is where the tricks come into play. At the fasteon (female spade) connecter at the speaker you will have the wires from the amplifier. You will also shove another set of wires in there which will go to the second speaker on each side. Wire those to the positive and negative sides of the second speaker just like the first. The speakers are now in parallel and acting as a single 2 ohm speaker instead of a single 4 ohm speaker.
So...amplifier output now. The single pair will be running at the rated 4 ohms stereo output. The pair 'o pairs will be running at the same rate at the 2 ohm stereo/4 ohm mono rate. For instance...if the amplifier is rated to put out 75x2@4 ohms stereo (RMS, mind you) then it will probably do something like 220x1@ 4 ohms mono which equates to 110x2@2 ohms stereo (the way you are wired). Now divide that 110 watts by 2 since you have two speakers on each output and you get 55 watts RMS to each speaker. Alternatively you could just divide the mono rating by 4. I just want you to see where that number comes from...division of two because we're going stereo and division of two because there are two speakers on each side.
55 watts to 4 speakers > 75 watts to 2 speakers in terms of brute loudness.
The best part is that you will still have stereo sound and a working fader to balance the sound so long as you run an RCA pair from both the front and rear outputs of the headunit!
If you choose to run the sub amp in there as well you will just directly wire each pair of speakers to an amp terminal since you will have a total of 6 outputs. The RCA patch cables will run with one set to the two channel amp and another set to a splitter feeding all 4 RCA inputs of the 4 channel amp. In essence you will have three 2 channel amps then.
I hope that helps. The math has the potential to be daunting, but really isn't. If you have anymore questions just post up and I will try to answer them for you.