Welcome to the Foums! Glad to have you aboard.
I can't help you with the "plastic problem", but I can give you my on the other two items. First, there are sites that you can go to to download copies of the instructions and there are members who have copies and can e-mail them to you and help you with them. I don't remember the sites but I'm sure someone on here can help you with that. As for the decals, when I get a kit, I take the decals out and place them in a zip-lock bag and put them in a file in my file cabinet. If they are old, the first thing to do when you are ready to use them is to spray a couple of light coats of Testors Decal Bonder on the sheet. What it does is to seal the decals and any cracks that they may have. It will turn the sheet into one whole decal, so you will have to cut out each individual decal before using. This has worked well for me over the years and saved many decal sheets. If the decals have yellowed, put them in a zip lock bag and tape them, good side out, to a window that gets a lot of sun. Leave them there for about 3 or 4 days and you will notice the yellow disappearing.
I would say that you could use the old brittle kits as learning experiences. If they turn out OK that's a plus. If they don't, it's still a lesson learned to help you as you build the better kits.
Ask your question about the AFV tracks in the Armour section of these forums. Someone there might be able to help you find replacements.
Sit back and enjoy this great hobby. We'll be here if you need help.
Jim