I'm looking for the best way to animate a line that needs to match a text box length

Here's a highly customizable effect I created; for creating a typing effect.

http://www.broadcastGEMs.com/typeonGEMs

As for the underline effect, you'll have to look into using either the sourceRectAtTime Expression Method or the newer Create Nulls from Path Script - but even then I'm not certain if it's the most efficient method, on its own. You'll have to combine the effect with the Type Tool's Text Animator, to ascertain which word in the text layer will be underlined. There is also the case of more than one word in a single text string having to be underlined.

You'll need to specify clearly what you want because scripting or writing Expressions works best when the problem is clearly and correctly stated - you've not provided sufficient details, to this extent.

I suggest you post your question in the After Effects Scripting Forum, after thinking more clearly what you want; especially in terms of what the line looks like and how you want the words to be identified, to be underlined and how frequent this underlining will occur..

After Effects Scripting