saturation

I am building some pages out and was curious about keyword density. For example, if my high target keyphrase is "Elliptical Machine" with 600 words on the page, what would my guidelines be? Other related phrases that are highly searched are "Elliptical Bike" and "Elliptical Equipment" and...

I'm thinking of the diet niche for my blog. Has anyone tried to create a blog in this niche and have been successful? (300+ $/mo.) I'm afraid that this is a over saturated niche and will be flooded out by competition thoughts?