I hate to say it all comes down to experience...but it really does.
On major systems consider age, stroll youtube or manufacturers websites to learn how to estimate the age of an air handler or furnace. Get the first folks that estimate a roof for you to demonstrate how to gauge the life left in a composite shingle. Learn some of the big warning signs in regards to electrical (aluminim wiring or older knob and tube....not that I see much anymore but it is out there, that stuff will require replacement most likely).
You can look at a floor and tell me if its serviceable right now. Its easy to measure the square footage and get an installed quote from any number of suppliers, home depot offers this in most markets.
I don't know what your partner meant about outdated walls....and that's completely subject to my budget and intended use, but rarely would I find the average house in need of full on wall replacement (though it does happen). I'm more concerned about how much patching to do, water damage, anything funky that a hack re modeler has done, that sort of thing.
Again, it depends on my intent, but I wouldn't move to replace windows unless that fit within the scope of my intent. I've bought and sold houses that had perfectly serviceable wooden windows.
If I was 20 and starting out again I would find a place to learn. I did this by volunteering with habitat for humanity. In my area that was a bunch of knowledgeable old guys that were glad to have my then strong back. I learned the basics of electrical installation, drywall, and framing that way. I only went when I had the time, and I met some useful contacts.