Using the 4runner in a northern state as an example of rust problems is kind of choosing the worst case example, no? The model has pretty much always had rust problems in the north, unless you are very aggressive and regular with anti-corrosion applications (water-displacing oils, shielding clays & polymers).
Until just last month, I was driving a 20yo 4runner - when the frame did fail from rust. The rest of the truck was in perfect condition though (even the upholstery on the driver's seat was intact and original - seams on the 3rd gens usually failed in the first few years), and I did consider dropping the ~10k for a frame swap and mechanical overhaul. At least for a hot second.
If someone is doing an EV conversion on a car twice that age, safe bet they have not only taken steps to take good care of the car, and that they are an enthusiast to some degree.
EV conversions aren't for you mom's daily-driver econ-o-box. They're for someone who wants an EV without buying a new car, or for someone who wants to do it for fun.