>in addition to this on every movement there is something else involved: chaos. if i remember my physic classes correctly this is teached by Quantum Physics.

Somewhat right. Quantum is about uncertainty, not chaos specifically. Even classical, simple systems may demonstrate chaos. See for example:
http://www.wolframscience.com/summary/

>In real life (our solar system again) this would translate to minute differences on the path/speed of each planet. now of course they will always travel around the sun, but maybe the speed will change just a tad.

And that is accounted for in the model. All objects with mass have a gravitational pull.

>So the bottom line is this: what are you trying to accomplish? If you want an exact reproducible result, a theoretical model, I think your solution is the more appropriate one. If you want to be closer to a real life situation I think my outline is the one to go with.

I want to create a model that does both. :-) Once I have a good working model of Newton's gravity, I have some ideas to make it closer to reality (relativity and beyond).

>>For the purpose of physics, a determinate model would definitely be more valuable. I think. :-)
>
>what is a "determinate model" (english is my second language)

Runs the same way every time.