Detuning means taking a file to the edges to reduce the "catchiness". It's a bit of a religious issue. I personally own a Heritage and I've never detuned anything, and it hasn't hurt me. If you don't know what you're doing, don't do it.
As for wide vs regular board, a wider board means more distance from center to edge, which means instead of your toe (or heel) pushing down over the edge, it's pushing down before
the edge. This increases the amount of effort required to put the board on edge -- maybe by only 5 or 10 percent, but by the end of the day you'll want
that 5 or 10 back, believe me
It also changes the way
you edge, since you'll have to put more effort into lifting the heel (or toe) to compensate.
Anyway, my first board was a Morrow Lithium. When I got the Heritage, I discovered the difference between the Morrow and the Heritage was significant. And it did take some getting used to. But as for "too much board", think of it like the difference between a Chevy Impala and a Ferrari. It's hard to get in trouble with the Impala because you just can't get it going that fast. With the Ferrari you can kill yourself a whole bunch with little effort. But if you respect the power of the Ferrari and drive within your limits, you're in no more danger than you would be with the Impala. Rider > Board, as we say. So get the NS, realize you're getting a board that can take you down the hill way the hell faster than you can handle, can turn you way faster than you're ready for, and just take it easy while learning.