Need help: Is this car's undercarriage in bad condition? Looking to buy soon.

That specific vehicle is missing the aero covers, all trims have aero covers.

 

My thoughts:
-It doesn't look like you're missing any plastic shields; we can only see the very front of the car in that picture, from the catalytic converter heat shield forward
-the two circled parts are: lower) suspension lower control arm, upper) steering arm (sorry, I'm blanking on the name of that right now!)
-that looks to me like just dirt on the frame, I wouldn't worry about it

 

For reference, here's a picture of the Gen 3 underside:



The part we're looking at in the OP picture is that open area behind the front wheel (disregard the x; I stole this picture from teh interwebz).

 

You can see the very front of the RH floor pan cover in your first photo; so, no it isn't missing, and I would bet the other isn't either.

 

I can't imagine why the bottom panels would not be factory installed on a level 4 Prius. My '14 level 3 still has all the panels including the oil flap door at 170,000 miles. Perhaps that is the story he is telling you since replacing the panels would be dipping into his profit? If that was the case it might make you wonder about the true value of buying a certified used car.

 

i don't think so. look on car parts.com, one of them, the 'engine splash shield' is only 28 bucks

 

15 gets new pistons and rings, i hope that does it!

 

Good luck. You are correct that it is very hard to find a rust-free vehicle in the north east. No matter what I do, my cars always show some problem related to rusting by 10 years old.