I don't think you need to have a full Club Race cage or anything to run DE events for PCA Potomac. As long as you pass the inspection I think you are fine. Contact John Brown or Don Newton for the specifics...you can find links to their e-mail at:
http://www.pcapotomac.org/pca/html/deguys.shtml
Helmets are another story, for our region I am almost positive you can't use a motorcycle helmet
I think they want us to use auto helmets because they are better in multi-impact hits. I just got a nice new helmet for my bike and am bummed I won't be able to use it. You can usually borrow one from someone if needed at the DE though.
Nathan