I think you're on the right track. When I started collecting, an M15 was $50 and considered expensive by the older collectors. Tony of Kaiser's Bunker had some great advice about collecting. Start by deciding what you
really want to add to your collection and make a list. For example, you could indeed try to collect an M15 from every state, Duchy and such. You can concentrate on branches of service, Infantry, Artillery, Kuerassier, etc. You could also collect a specific unit... which is very challenging. However, making a list of what you want first is a great idea. It keeps you from buying helmets that won't fill a hole in the collection (almost impossible for me to do) and saves money. Books are also a good investment. I also browse web sites to see originals for sale, because there are often excellent reference photos.
Money. Yes, there's never enough for me either. If you're married, keep the wife in mind, because it's her money too in many cases. My wife has been more than understanding with this hobby, allowing me to spend the cost of a new Harley over the past few years. (which is why I don't have a Harley!). Now, the spree is over and it's back to saving. So budgeting is very important unless you have a huge bank account... in which case you'd probably be collecting tanks, warbirds, cars or something. :wink:
So: 1) make a list 2) research books 3) keep the wife happy 4) budget and save. 5) ENJOY the collection you have, even if it's only one helmet! (there are also photos and 'ephemera' to collect... but again, money!)

Ron