mlts22 makes all very good points.
I reinforce that in general, mechanical problems with the chassis will be easier to handle than structural problems, and with a much more successful outcome. With that said, when shopping around for a used rig, "Low Miles" is not necessarily a good thing. It would depend on how the rig was stored over the years. If it sat outside and rarely moved, that can be very hard on the chassis. But in heated dry storage, the rig stays very well preserved, just like a collector's car. Some of us call those rigs "Cream Puffs". They are hard to find because the garage needs to be unusually tall and deep.