I think the best approach for trying to get these shipping rules in place would be to start with this first one and work out a little more about the specifics. You assessment about totaling the weight for all of the items is correct, this can be setup as a trigger for the shipping rule. I'm curious about the other requirements for this first rule, like defining which region the order is being delivered to since this will affect the charge.
The other thing about this charge is making it more for an incremented amount of weight like 5kg. This part will be a bit trickier to implement as it will not be supported within the eCart interfaces and must be customized directly in the eCart code. Please post back with some more details about this first charge rule and I can review to give you any pointers or advice on implementing it.
Also if you have not done so already you should take a look at the eCart support page and examine the documentation that has to do with discounts and charge rules, here is a link:
ecart/
