Searching for the best materials can be misleading, it seems. Don't try to be too smart about stuff that is dirtcheap and works. Designing the parts and make them fit together is what takes time.
Injection molding is for standardized and mass produced parts. As long as one doesn't mass produces anything it's most likely not relevant.
If you get it cheap, including shipping costs, then maybe it can be useful (for prototyping), but it's not going to be as strong as cf or aluminium. It's not being used as much as 3d printing recently for good reasons. If you can't get it very locally, why would you even leave your house and go somewhere and spend a lot of time to get it? I don't blame or mock people for being broke, but PLA costs maybe 12$ a roll and it can be printed in sheets. You can print it in the right size and form, no cutting needed. Some fumes, vs dust which might require a workplace outside of the regular living space.
Also, most plastic parts might be molds or the bones, which are shaped in all kinds of irregular ways. Some spare plexiglass or acrylic won't be very useful there. Aluminium tubes and parts are cheep as well and could be used instead plastics. If PLA is not strong enough then build a enclosure for your printer and use some special Nylon or even better: PC.