I always send our clients to buy their licenses from the Oracle Store; it's the easiest way to do it and saves them from getting repeated followup calls from Oracle Sales trying to sell them more stuff, which is what happens to the one client we have who went through a sales rep.
My read on the named user thing is that in order to be legal you have to buy enough seats to cover the maximum number of users you exepct to have on your site at one time. For your Intranet that's calculable; for a public site it's kind of tough to predict. Of course you can probably get away with just buying enough for the number of nsd threads you configure, since the internal "high water mark" table will be correct, but you were asking about doing it right so that doesn't count. :) Doing it right it's easier and probably cheaper, unless your site's audiece is very small, to just buy a processor license and be done with it.
I have never called Oracle support; everyone I know of who has has been underwhelmed. The first line support people seem to be the new hires and many of them don't speak English very well, which is always difficult on the telephone. What I hear is that you have to convince them to escalate your issue up several levels to get someone who really knows how to help you.
On the other hand, their Metalink site is quite good; you can search the bug database, articles that their Tech Support folks use, and open help requests. I've found lots of good answers there, and just for that reason I think it's worth paying extra for support.