Hydroton is a great medium if you dont mind cleaning it. Most people would agree cleaning the hydroton is a pain in the arse.
Coco is light weight and reusable provided you take the right steps in conditioning the coco before reuse.
The placement of rockwool in your pot is personal preference. Some people prefer to bury it and others advise against it because larger rockwool cubes tend to stay wet too long, it all really depends on your specific setup.
The pre-made buckets have plenty of room for root growth and I have never personally observed a need for a bigger bucket (I grow 3 foot trees). Again, I would have to say this depends on your grow situation and preference. What works for you will not always work for another grower in their specific grow situation, do what works for you.
Throw it in a washing machine