Hi and welcome.
1. I use tubs but I am planning on upgrading to PVC come January. I love the look of PVC and it will be easy for me to keep husbandry up for my three snakes. I tried using tanks and just couldn't get the humidity right, even using all the tips and tricks suggested here.
2. As long as the breeder is experienced with shipping, it's no big deal at all. I had my brazilian rainbow boa shipped to me and she handled it perfectly.
3. I don't have a PVC cage but I've read here that 12" can be kind of hard when it comes to cleaning. I would just put the cage up higher, maybe, so you have better access?
4. My bp's have proper heat but I don't give them extra lighting and they do perfectly fine.
5. I use paper towel and swear buy it. I buy the thick Bounty rolls. It's super absorbent and cheap, and it serves as an extra hide! I always find my snakes hiding beneath the paper towel since it's light enough that they can squirm under it. They love it! I wouldn't do a reptile carpet. With the amount that bp's mess up their enclosure, it would be a huge pain. You'd have to have backups so you can swap them out and clean the dirty one.