Would it be appropriate to run a 15x8 +0 offset wheel with let's say a 185/195 and/or 45/55? Not sure how stretched the tires would be, and I don't know much about the subject and how it would effect the handling.
NO! #-o I get the feeling that you have never auto X'ed before. Size is meaningless if you dont have a good gripping compound. Not to mention stretching some piece of crap bicycle sized walmart tire (Like Some) over a 15x8 wheel isnt going to help your case at all. If your just starting out go with a set of Azenis 615 in say a 205/50/15. They are cheap and decent in auto x. Once you get some experience under your belt and decide you want to stick with it you can upgrade to a nice R compound tire. Also I wouldnt run a 15x8 +0 unless you have your suspension set up properly and a very good alignment. You should also read the rule book to see what class you are going to fall into. A 15x8 wheel would bump you in class right off the bat.
If you have any more questions feel free to ask. Sid also made a good point. I run class C-Stock with my NB and use my stock wheels with a set of Kuhmo V710's. In the class you have to run stock sized wheels. Stock NB wheels are like 12lbs and change so unless your like super uber omg I need every little advantage then its not worth spending like 1200 on a set light 14x6 volks or something similar.