Stereo system. Cheap and cheerful 4.1 setup

I only scanned that quickly but if the average (rms) power is 570Watts then your alternator is not going to be enough if you're going to wind up the volume given that its under 100 Watts.

Looking at the link I would be inclined to save up a little more and get a bit better kit. I have a 4x100w head unit and a 150w under seat sub with some suitable spec mid price speakers and I only spent an additional 35 ish quid for a system that is loud without being chav to the outside world.

Also with a system of 550w even at full bore it puts less load on the alternator than having full demist and rear screen heat and lights on, judging by the way the idling changes which is not science I admit.

Oh and the alt is 70 amps or 1000w at 14.4v

When it comes to sound systems, wattages always need to be taken with a pinch of salt! Especially when it comes to head units. Regardless of what manufacturers claim, 4x 25 watts RMS is the realistic useful output of any 1din headunit given the restrictions on space, cooling and wiring. Hence my suggestion of an amplifier for the front speakers. A £60 Sony 4 channel amp will be 3x as powerful as any headunit.

As far as alternator draw goes, even if you did have a system that could exceed the alternator's capacity, you wouldn't want to have it at that power level for any length of time if you were sat inside the car! An actual 300w of noise in a confined space like a Jimny would be pretty unpleasant, let alone the 1000+ watts that the alternator can generate.

If i listened to my Stereo at 3/4 volume it would give me a headache

Going off rms mine is 4x40 continuous at 14.4v which for a single din is currently as good as it gets. By the same token I have a spare amp that is rated at 4x50 rms and I dare not fit it in the Jimny for fear of making my ears bleed. So much is marketing bs that without calibrated test equipment it is impossible to know for sure what you actually get.