Long time 2boxer New to 5boxer

[Khatovar]

So should I first make the 1xpala (which I got already) and 4x Shaman at start, then slowly start with other combination to get the hang of 5boxing?

Probably, you're likely to start over a couple times anyway, learning more each time. So start with an easy combo so you can figure out the basics like macros and such on just one or two classes. Once you get comfotable with that, then you can worry about more complicated group makeups.

last account I got it from a friend who no longer plays and deleted all of his chars.

That a big old no-no in game and thus on these forums. All the accounts should be in your name, otherwise you are violating the ToS which states you may not sell or trade accounts. Doesn't matter if he deleted all the toons, go buy a new account.

Keyboard. Should I get a G11/G15 keyboard to make my life easier? Or should the standard keyboard would be enough?

I have used a regular old keyboard for years. The other stuff could be easier depending on what you do, but I haven't really come upon a situation where I've felt I "needed" or would have done better with a G15 or whatever. Now, a mouse is a different story, I'd love a mouse with a few more buttons, but my boring old Logitech MX400 does me just fine.