Replace a Front Frame Rail - How Much Trouble?

Rule #1: The damage is going to be worse than you think.
By the time you cut that back far enough to get to solid metal to work with, you'll basically be putting in a new frame rail anyway.
Get a copy of the body/frame section of the FSM, so you will have all the dimensions for squaring up the new section- a car that has been gradually rusting out has also settled over the years and likely will not be square any more.
2.) Be prepared to replace more panels than you thought. An adjoining panel may seem solid, but the lap seams have rusted along with the rail, leaving you no way to reattach the panel. See rule #1.
It's not for the faint of heart, but don't be intimidated- and be honest about your own abilities. It's just a large jigsaw puzzle that gets welded up when you're done.
Take it for what it's worth- from a poor Wisconsin rust belt guy who has seen WAY too much of this ...