You know, maybe for sake of keeping gears cheap and still getting out there (which is what really important as well as being safe.) , is it possible to just camp at campground, and go hike trails and back to campground? If the cheap gears fail or something happened you at least have safe margin of the car with ability to seek help.
First time I ever went backpacking, the temperature dropped to 32. I was using a "30 degreed rated" Coleman sleeping bag that my brother had. I shivered all night long....
You mentioned you didn't want camp in temperature below 40. I was hiking during first weekend of June. A little rain came through and at 6,500 feet the temperature dropped to 36 degrees.. IN JUNE!! I can imagine the mountains in Idaho be just as cold during summer months at elevations. So, that why I'm thinking maybe do some camping at campgrounds in national parks and forests, and explore trails with panoramic views then hike back to campground.. Just getting out, hiking all day, stay within safe confine of campground until your son's health improve and your financial situation improve, then you can advance. Also, this may not be ideal, but you can rent gears. Depending on vendor, they may require credit card hold incase equipments get damaged. So, maybe suffer a short-term loss to see how backpacking will play out in long-run for your family before splurging.
It is one of the blessings of wilderness life that it shows us how few things we need in order to be perfectly happy.-- Horace Kephart