Took a peek at the listing... the truck looks right. I love it! You COULD buy a restored/finished truck for similar money, but you will NOT have a truck that is unique. As they say... They are original only ONCE! Go try and find another!
As for the rust underneath, some of the parts had no finish on them when new and began to oxidize on day one. Other parts only had thin chassis paint, and age shows. It's called patina. Actually, a lot of folks are trying hard to fake that look nowadays. This is REAL!
Imagine if the exterior of the car was painted the same way, then was splashed with twelve thousand miles of dirt, mud, water and so on, and never washed. It would look like heck too.
If you have a soft spot for this era truck, and you appreciate originality, then consider this truck some more. Buy a plane ticket and go look long and hard at it. Examine the parts the guy took off and see if it all checks out.
I prefer later trucks and found/purchased a '66 with only 8800 miles on it. Almost exactly the same story. It was in a garage in WA for every day of it's life and basically unused for the last forty years. The original owners family had prepped it for sale and unfortunately had done some things that should have been left alone. A pressure washer can do a lot of damage in a hurry in the engine compartment. Old engine paint just blows right off along with the dirt and oil.
Anyway, I have been admiring trucks for all of my adult life and nothing comes close to a highly original survivor. You can have all the glossy, restored and upgraded ones as they number in the millions and do nothing for me anymore.
Do your homework and you might end up with that '49 black beauty. If what you really want is a shiny truck with lots of billet and braided stainless, then leave this one alone!