I don't know about your specific needs in particular, but here are some general answers.
1) It all depends on the specific hardware of that system. Likely there will be no problems, but you should do some research to see how well it's supported first. There are many Linux "live cds" out there that provide a good way to test things out before you take the installation plunge.
2) This depends on the individual programs you want to use. If the format of the data is well supported by the Linux equivalent, or can be converted easily, then you should be able to transfer them.
(ETA: Formats that are based on common, open standards are generally well-supported. Support for closed, proprietary formats ranges from average to not-at-all.)
3) Same answer.
4) The status of documentation varies, especially online. Sometimes you're overwhelmed by information and sometimes you have to search high and low for what you need to know. And sometimes it's outdated when you find it. But the general stuff and popular programs are well-documented, and there are always forums like this if you need help.
5) Linux is a completely different operating system, so in essence, none. However, there are ways to work around this. a) you can set up a dual-boot system, so that both operating systems co-exist on the same computer and you can switch between them. b) you can run one OS, and its programs, inside a virtual machine on the other OS. It's generally recommended in this case to run Windows inside of Linux, instead of the reverse. c) Many Windows programs can run under wine
, with varying results.
6) In general, a Linux OS will run much lighter on the same hardware than the equivalent Windows installation. So you shouldn't need to buy any new hardware (assuming point 1 works out ok). Of course video editing is a processor-intensive process, so the more powerful the hardware the better.
Finally, some advice. Linux is a great OS, but it's not always an easy OS, especially when you're just starting out. Be prepared to spend some time and effort to learn it. Expect some frustration and confusion. But once you do know it, you may never want to go back.