I myself am in the same predicament as you. I have a 99 3rd gen limited (237k miles), 03 Sequoia limited(230K miles), and a 2015 GX460 (72k miles) premium fully loaded. I was considering getting letting my 3rd gen go for a gx470. Found one with low miles, but frame rust was questionable. I saw it in person and didn't look to bad. Pictures really enhanced it. Hearing of the 120 platform starting to have experiences of the Tacoma, sequoia, and tundra crowd, I decided to compare it to my 3rd gen and sequoia. 3rd gen still looks good. No rust prevention other than occasional undercarriage blasts at the car wash. Sequoia was looking to be in better shape until I just found holes in the frame just above my control mount on the driver side. I bought the sequoia almost 4 years ago. Knew of the potential of the frame but thought I could get at least 5-6 years out of it. Apparently this is not the case. The rust snuck up on me quick. Thankfully I only paid $5700 for it and got 50k miles out of it with no issues at all. This experience alone has made me change my game plan. Up until then, I would looking to see if anyone was able to salvage a frame by removing the rust scaling and POR15 and/or fluid film treatments. There are a lot of opinions on what works best and how to use those products. Now I am looking at a sequoia replacements in the south and west instead and undercarriage sprayers for pressure washers.
From the pics it doesn't seem bad, but from what I hear, the frames are know to rust from the inside out when not cared for. I would suggest doing research about what a bad frames look like and depending how long you want to keep it, rust converters for frames, and then decide for yourself.
