Well, if you want a reeeealy nice helmet, and are willing to spend alittle extra $$, You probably wanna buy a Fett Pride helmet. this is a great beggining helmet, comes unpainted and with all the bells and whistles. If you want something on the cheaper side, but not as nice, you can try a cheap Rubies Boba fett Helmet and repaint it.
Also, It'd probably be best to do a rough sketch of the mando you plan to make, just so you have a basic plan to work tawords. Then again, if you just wanna go wild and let the creativity flow, you go right ahead, man! That's what this hobby's all about.
Best of luck.