I recently bought a used Expedition but in the search process looked at the prices for Suburban, Armada, Sequoia, ect. While the Sequoia might be the best of the bunch, you still have to pay the used Toyota mark up. If you are looking for a bargain, you are going to have a lot harder time finding one with a Sequoia than the other brands. You might also get what you pay for too. My Expedition has been a POS.the sequoia is plenty strong, an since they all share the same frame I wouldn't worry about anything. the 200 has higher tow rating by 600lbs than the sequoia due to the sequoia being larger and heavier. I personally wouldn't be questioning the strength of the frame with all that said.
there isn't an inherent strength difference between a solid axle and independent, it really just boils down to engineering. Generally independent suspensions are not as heavily reinforced as solid axles because they are not supposed to see the level of strain a solid axle is meant to. That said, Toyota of all brands, actually engineered a really solid IRS system for the Sequoia, to the point that it was considered for while of dropping the solid axle from the Tundra and leaving it IRS like the sequoia - plus the ride is much improved. Obviously that idea never came to fruition.
If you are buying used, the Sequoia is a bargain. Thirsty SUV's took a huge hit in the early 2010's with high fuel prices, and their resale never recovered. now would be the time to pick one up.