Your head unit is 55w x 4 (Peak). You are probably about 22-30w RMS.
Is $200 your max budget for everything?
What size speakers do you have?
Why does having an old car mean you don't want to spend money on the stereo? If you get a new car, then take the stereo out, and take it with you.
Your 3 options are
1. Buy an amp. Even a used one that's about 100w x 2 rms. You should be able to find one for $50-$100, then use the rest to find some decent speakers that are about $100 and 100w x 2, and save about $30 for wiring. You can find $50 speakers that will play off 100 watts, and you can find several hundred, and thousand dollar speakers that will play well off 100 watts. This way you get it going, and if you ever want to upgrade, you just upgrade the speakers.
2. Buy a different head unit. Your head unit is 55w x 4, so probably half that RMS. Find a head unit that is 50w x 4 RMS. Then you can find 4 speakers that are 40-60w RMS and you are done with it.
3. Buy speakers that are about 25w RMS. They won't be much of an improvement unless the ones you have are already damaged. Could probably even buy used speakers out of another vehicle that are a better quality, (but don't expect much unless they are some special name, and if they are, they are probably going to cost more).
I don't know anything about Spain, but here I could always find a small 2ch x100w amp for $40 or $50 all over the place. If you have
ebay, craigslist, or any car meets or car show, you can always ask, and buying used you have a better chance of getting a package deal. Better chance on getting the wiring included, or a discount for speakers and an amp if they have it.
When you say 1:2. RMS is USUALLY half of Peak/MAX, but not always. Match up RMS numbers. Don't need to be exact, but 10% is no problem.