You've already gotten a lot of opinions, but I have to say that it looks like you'll need a lot of clean-up at the very least, and maybe even some safe-t-caps. But if you're lucky, you may find as you clean it up that it's all mostly surface-level, and is evenly distributed across wide areas (not too deep or too bad in any one spot, making it weak).
There are lots of threads on this forum to guide you, because this is not an unusual problem. Search around a bit.
For inside the frame, consider drilling drain holes in the frame, and then investing in a sewer jetter that you can attach to a pressure washer. Once you've cleaned it out, a $40 scope on Amazon can give you a peek at what the inside really looks like.
Don't forget that the transmission skid is a notorious spot for hidden rust, perhaps second to the inside of the frame.