Welcome to the forum, and congrats on your upcoming LR3 build. Enjoy the journey - I’m certain that you will learn lots, have a few frustrating (learning) moments, and have a lot of fun along the way.
Sorry, I can’t help you here, but I’m pretty sure that someone has used aluminum to build the table frame. Maybe do a search of the forum
Not that I’m aware of, but there are plans available on the forum to build your own (here). Many people have used their LR3 to cut out their torsion box table, and it brings a certain satisfaction to cut it yourself)
No, nobody sells them, as shipping would cost way too much. Again, it is very satisfying to cut your own with your own machine, and there are lots of customizing options available.
Are you talking about the XZ and YZ plates? There are DWX (CAD drawings) files available, but you have to do your own CAM to create the gcode. EstlCAM is a great tool to learn
Edit - LOL, I see that @DougJoseph beat me to the punch and answered all of your questions better than I did.
Doug is a great resource on this site, and has created a lot of customized remixes for certain parts that are really useful. Check out his Printables page.