Here's a few pictures from down under (no offense Aussie bros). What am I up against...and how soon?

Looks about normal for a Datsun. Frame and floor board replacement is about normal for our cars. Look under the battery and for rusting coming from the inside to the outside. Depending on how good you are with a welder (mig or tig), it will tell you how hard and how long it will take to replace them. Then sit down and evaluate how much you want to spend? You can end up spending a small fortune and a lot of time fixing it. Be honest with yourself and decide what is your spending limit is. A lot of projects get started and never finished because of this. It can be fixed, any rust can be cut out and replace. It takes a lot of time and hard work. This has been known to start small and go into something real big after they get into it. You are the final deciding factor in doing it. I have over 4 years in auto body repair and would do it. But you may not have the skill or the resoures to do it. Good Luck with it, Rich.