I was also in the same situation - I briefly looked at the undercarraige on my 05 SE V6 with 114K miles and the rust was comparable to the pics posted on this thread. No dents or dings though.I wish I would have taken more time to inspect the undercarraige of the '03 I just bought last month. Granted, I took a quick peek for drips & leaks while running, during my test drives.... but I must have just been using my "stupid" eyes that day, because I didn't even think to consider or notice the rust. Like I showed in another thread recently, my tow hooks are rusting completely off in the front, and the radiator support portion of the frame looks really bad.
Now, I'm considering selling this and looking for something else, maybe even a Sequoia... I've been looking at an '03 Sequoia that's about 150 miles from me (online) but from what I've been reading lately, the Gen-1 Sequoia's had a Frame-Rust epidemic (like the Tundra's which were recalled due to frame rot), and until I can inspect or at least see pictures of the frame on that Sequoia, I can't justify making the drive for the purchase.
I had rust-killer sprayed on - it reacts chemically with the rust and makes it inert. I then sprayed on a rubberized undercoating. It looks a lot better now, but I wish I had shopped around some more. I keep looking at other newer 4Runners and may get rid of this truck in the summer and get a newer one from one of the states mentioned before (TN, Northern GA, Arkansas, Texas) and have it shipped to Canada. I've seen lots of low mileage 07's and 08's in what appear to be excellent shape and CPO'd from Toyota dealers for less than $23K.