I don't think there is any harm in buying a helmet, especially if you go with an MRCE. You could always modify it later.
Personally I have the problem where I get "hooked" on an idea, where I think something is cool and decide I really want it, but once I get it/have it I hardly use it. That's why there is a 1969 Volkswagon Beetle sitting in my drive way that I hardly drive anymore. I saw it for sale on the side of the road one day on the way to school and I said "I NEED THAT!", so I went and bought it. Probably the worst investment I have made so far in my life. I thought I really wanted it at the time but now its just sitting there.
Stormtrooper armor is really expensive if you only plan on using it once or twice. If its something you plan on trooping in and using frequently then I would consider it a "good" investment. If you want to wear it to school once or twice, or use it for a Halloween costume, only to get bored with it and have it sit in your closet then I would consider it a "bad" investment.
If you buy just a helmet then at least you are only in the hole a few hundred dollars or so. That's basically what I did. I bought my MRCE helmet when they first came out, wore it with a white tuxedo for halloween (TK-Suave ), and now a year later I am modifying the helmet and investing in some armor.
So yes, long answer to your short question, I think its a good idea to get a helmet first, see if you really like it or you get bored with it, and go from there.