Hi DL, welcome to the forum.
To answer your questions
1) The route is very easy to follow. I didn't have a map or guide, I just followed the yellow arrows and always got where I was going without any problems.
2) Its very easy to live out of your pack, this forum has many packing lists of what you'll need and also discussions of what you don't need, but you can live easily and comfortably out of a 10 kilo pack for eight days, so long as you do a bit of clothes washing as you go along.
3) San Sebastion to Bilbao is easily doable in a week, your pace depends on how much you want to walk each day and how fit and used you are to walking.
4) The tourist information, and the alberge will have information, basic guides available, and you should be able to get your credencial there, or you can get the credencial from this forum.
5) In September it is unlikely you will need to book accommodation in advance. The hospitaleros in the alberges are usually very well informed about the next days walking and can tell you what your options are for the next nights stay are.
6) I had a budget of 25 euro a day which made it possible, it usually averaged out to this amount each day.
7) If you are not going until September you have plenty of time to research your trip, check out some of the threads in the Norte section, and look at the eroski and gronze websites, there is a wealth of information there.
I loved walking the Norte, it is a fabulous route and if you enjoy it half as much as I did you'll have a great time. Happy planning and buen camino!