IMHO, the K-70 feels "very solid" indeed - so I would not have any worries about that aspect
PS: I think that referring to the body as mainly "plastic" is rather "misleading" because that word covers " a multitude of sins", i.e. everything from brittle stuff that breaks when you just "look at it" to other stuff that a hammer will bounce off if you hit it hard - and the K-70 body material looks/feels much nearer the latter than the former
PPS: had a few "accidents" with my Pentax bodies, and the latest of those was last year on a river cruise on the Rhone - the K-70 had the old Sigma 17-70 on it and was in the zipped small top compartment on my LowePro Flipside backpack. Whilst still on the boat (luckily!) I turned around rather sharply - and the camera and lens flew out of the backpack and hit the carpeted floor about 2m away because the I hadn't done the zipper up completely.
A couple of "sweating" minutes and several experimental frames later, I concluded that everything seemed to be fine, as has been confirmed over the following few months (No, make that nearly a year!)
Pentax make tough camera bodies, and Sigma make (or at least used to make) tough lenses