#1) Rubies is Okay for a first timer, but if you want something good, there are plenty of people here that make their own. Rubies take a lot of time and money above what you pay for the bucket to make them look halfway presentable. If you're like me, you'll probably see the value in letting someone else do the work and pay the extra to get a completed hat.
#2) Sintra is the easiest to work with because you can use a heat gun to do most of the shaping. But, there's also ABS plastic and fiberglass as well. Someone here even cast a Mando suit from aluminum sheet metal. And again, the time and material involved might make you think about buying one pre-made.
Sintra is Condensed PVC sheeting. When you heat it up, it expands and becomes pliable and easily shapable. Found at most plastic retailers and sign fabrication shops
Rubies is a costume retail company. Most of there items are quite pathetic to say the least. The Jango isnt too bad but is extremely small. So unless you are small in stature, its gonna look out of place. All the other helmets are pretty much just lumps of vinyl.