Helical gears essentially increase your contact ratio. A contact ratio of less than one will crash and will not work, so 1.2~ish more of a minimum: here is a paper that shows an example of two meshing gears of one tooth each, but with a very high pressure angle and helix so that they stay in mesh.
Helical gears also generate thrust (axial) loads, so make sure that motors or other purchased assemblies can handle the thrust load
Herringbone gears (essentially back to back opposite handed helices) don't have the thrust problem, but cost a lot more.
But for noise, profile accuracy is vital.

 

oops - 1.02 is more of a practical minimum - dang shifted decimal point.

http://www.akgears.com/papers/direct_gear_design.htm

this describes all kinds of crazy gears