I figure the head is a rounded cylinder, but trying to model the raised edge that goes from the brow to the cheek is proving hard for me to make look right. Should I separate these components or model the helmet as one mesh (save for the breathing tubes attached to the grill)?
Just search images for 3d model 40k space marine: you will find nice ref and even polycount/Z-brush threads about the WIP… maybe also 3D printables (using as inspiration) for example:
but remember use the original photos for ref don’t remodel those models.