Greetings and welcome to the forums ;p (haw haw Rob beat you to it)

1) Far as System goes, high specs would be best:
A Quad-core processor, some say Intel is best, as AMD's quads aren't quite as good
If you plan to 5box on 1 machine, 4gb DDR2 RAM is the minimum to run 5 comfortably; any more is icing on the cake though 4-5 should be plenty ;p
8800 nVidia card hands down; whether it's a GT GTS or GTX or wtfever, I'll leave to the guys who know what that difference means; I do know 8800 is more than enough to work with, and I don't hear quite as many good things about the 9xxx series. Stick with what's out, stable, and well documented and probably cheaper :p
550-750w power supply should do you well
HD get a Sata, don't necessarily aim for the highest speed, WoW isn't THAT bad about it...I'd go about 7200rpm probably. There's also some debates scattered about the forums regarding 1 install vs 5 wow folders/installs same HD vs 1 or 2 per HD for speed increases; I recommend the first or latter, 5 folders on one drive = uber queue on your HD's seeking, not a major difference but noticeable enough.
Not sure if I left anything out lol; you can probably build a comp like this under $2k, possibly $1k if you do it yourself and are familiar with them enough and you sound like you may be :p I built my current comp for about $550 and I run 3 WoWs comfortably, AMD dual-core, 7900 GS card, 2gb ddr2, etc and that was a lil under a year ago, could be cheaper now lol

2) Depends on how familiar you are with the quests, it should take you about the same amount of time, maybe less, depending on you personally. My first 60s took 21 /played days, my Rogue took about 18, hunter took 13-14; the 2 65 boomkins in my sig are at 65.5 with less than 10 /played days. You'll probably spend alot of time at lower lvls getting accustomed to your new power and learning to wield it without blowing yourself to bits + setting up macros/adapting to changes, after that it shouldn't be much of an issue.

3) Wouldn't be jack we could do about it pretty much; we play within the EULA and don't abuse our focusfire to grief or harass or ninja others so we should be fine; people can bitch all they want :p Some of them DO know it's legal...but as I have to say often here and in general, people are stupid. So very, very stupid. And in those people's cases where they're told it's legal but don't care, they're selfish/retarded/both as well. lol; basically, we're not doing anything illegal, and while blizz could arbitrarily say we're banned, they shouldn't unless it becomes a major problem abused by griefers/botters/etc, which due to the time, effort, and costs involved, is unlikely to happen.

Amendment to 3) Boxing is not profitable for char/goldsellers, someone did the math and itcomes out to $2/hour, less than half minimum wage and that's without factoring in some setup costs :p And boxers are unlikely to dominate long at high end arena places due to the simple fact that CC for us is a bitch and CC is what wins arenas; possible but unlikely vs any team at any high rank