Working with values
 

I wonder if there is a stack of commands in the ui system which can be used for evaluation of specific numeric data like health or power values. Where can I find more information about that topic? To be more precise: For Example I want to create a function-like window which I want to sum up the total resistance values. Any idea?

A couple threads that cover relevant topics.
Conditionals
String contatination

You can perform simple math on any string with the usual operators (+, -, *, /). Keep in mind that all strings are coverted to numbers with 3 decimal places when performing math. Many UI element atributes won't accept numbers with float and there curently is no known way to remove float with SOEScript. Similarly there is no known way to split comma seperated values.

Regarding your specific project, any of the player mods that show a numerical value for health and power should give you a good idea how to get the number piped to the text atribute of an element.