Most places I run check your helmet. I don't know how the clubs in your area are but I would go with a M rated helmet anyway.
M95 or better means M95 or M2000 or starting this fall M2005.
Open or closed face is a personal preference and both work fine.
If you want the cheapest I would figure out your size and order something like this online.
http://www.helmetshop.com/items.asp?CartID=98-EVEREST-30649GRWLZ818&cc=TK110&tpc= 
