Glass > Plastic in my opinion. Plastic is lighter which is an upside, but I don't feel comfortable putting warm wort into it. I know it probably won't cause off flavors, but I'll stick to glass to be safe.
7.5 gallon pot is a great size to start with. I bought a 5 gallon pot, so I do partial boils. You should be able to do full boils in that pot. Also, just relax. There are hundreds of videos available online if you want to learn more about all-grain brews vs. extracts, or anything like that. Also this is what the forums are for! If you have any questions, just ask. Some people say to use the search feature, which is fine if you don't like that person to person chat, but if you have any questions, just ask. People seem to be very friendly on these forums.
Good luck!