Hi,
Any of those 3D tools will work to create the helmet.The only thing you will need to do is save the file in an .stl format so that the 3D printer can interprupt the file for printing. If you want to make sure your helmet will fit your head you could have your head scanned using a 3D scanner creating a 3D image of it and design you helmet around it .Look for companies in your area that do 3D scanning.Its quick ,painless an relatively inexpensive. The design can always be done first also and you could rescale it to fit the dimensions of your head
Next in order to build this in a 3D printer it will be expensive Anything that large usually runs into the thousands ...2000-3000 at least ....you might recoup some of you costs if you decide to make casts of it and sell copies.The cost usualyl depends on the amount of material used so wall thickness will be a factor when you come to build it .There are different materials it can be grown in and the the level of detail you require in it ... you might also look at a design that is modular ...you could save money in smaller builds of parts then assemble it .
Take a look at google sketchup... Its a free 3D program and has tutorials and is a intro level program from what ive heard to design.. there are other programs you can use with it to get your model ready to print after you have designed it ...CADSPAN
Lastly ...if this helmet isn't a unique design of your own but something fashioned after a helmet that already exists .. you might want to do a search on line to see if some one else has already mad a 3D mesh of it ...it might save you a lot of work.
I hope this helps you out ..
Tony