Ok for question number 1, no not at the moment RGH's and Jtags can't go online. But, people are working on it right now and are getting close to achieving it. A KV is in simple terms like the ID for the console which every single one comes with. You can extract the KV by using the nand reads and CPU key that you get when you have jtag'd the console or RGH'D. Unless you buy a console that was banned that someone RGH'd to use as a jtag or sell to make there money back.
2.RGH's usually cost less than normal Jtags because, Jtags were originally exploited on an old kernel(dashboard) where you only had to code it and put in a few wires and diodes to have it run in the hacked dashboard. They are very rare now, so like any rare thing they cost more. While an RGH is a newer console that has a chip installed. The chip with some files written to the console resets the cpu bridge allowing it to boot into a hacked dashboard. So they are more of them than normal Jtags.
3. They are a jtag, so yes they can run unsigned code they are just newer jtags setup differently.
4.Any console can play ISO discs you just have to flash the dvd drive.
5.Yes, like I said before they are just like a jtag just built differently.
6.Yes! That is a great deal since it is a 1TB hardrive preloaded. I would get it if you since you are new to modding. It would have probably everything you need to start off.

I hope this helped you!-FriedModz