I think it's best to just regard design-inference claims as parts of explanations, and evaluate them the way we generally evaluate explanations. Someone who makes a design inference infers that part of the explanation for some object's existence is that it was generated by a designer. So how would I evaluate this explanation?
First, I'd determine how plausible it was that there existed a designer with the capacities and motives sufficient for creating the object. Then, I'd evaluate this explanation on the usual grounds -- explanatory adequacy / simplicity -- against the best non-design explanations.
I don't know how to properly assess the claim that human beings and other organisms are really amazingly complex. (I don't know what a proper metric for the complexity of an object would look like, and it's up to the IDer to come up with a good one.) But when we start considering explanations including a designer who would have all the right motives to make us, just as we are, and all the weird species in the world, just as they are, we're going to need a terribly non-simple explanation to account for all the data. The designer will have to have all sorts of complex motives and mental states. To quote a Joan Osborne parody from some years back:
What if God smoked cannabis?
Do you suppose He had a buzz
when He made the platypus?
When He created earth our home?
Does He like Pearl Jam or the Stones?