I'm in the same boat as you, ish... suspected fibro, lumbar spinal stenosis, hypermobility, facet joint syndrome. All being "managed" with meds, as I'm past help...I'm 35 and on the scrap heap lol
Anywho, I want to start with the trying to build muscle, and , like you, gym membership is not an option, I'm on benefits, and they only just cover the essentials.
I am looking at maybe getting some of those weights you strap around your wrists or ankles for when you work out. They are quite cheap, and I can wear them when I go hobbling to the shops.
Don't know if you've thought of anything like that? maybe just use canned food as weights, carry a heavy bag whilst working out, obviously not so heavy you hurt yourself!!
Gentle hugs from a fellow achey person x

for the plantar fascists - good arch supports - and when in bed ,before u get up , lay on your back and put your feet at right angle toes pointed toward the ceiling that will stretch those muscles before you stretch them by walking on them and that what hurts .. they cramp up during the night.

http://www.nerdfitness.com/blog/2009/12/09/beginner-body-weight-workout-burn-fat-build-muscle/
This is a good site for body weight exercises which is recommended a lot. Also look up your body is your gym