https://www.carid.com/2018-ford-focu...847877573.html
I really want to buy these custom projector headlights and install LED lights in them but i'm wondering if adding these + LED headlights to my car would be legal.

 

Not sure what the issue would be, but I can tell you first hand that the modern headlights on cars out here sure seem to be brighter.

This is especially true for the Audis with the LED headlights.

As far as I know, the only issue for lights is if they are different colors.

How bright are they?

 

As long as they are "FMVSS 108" compliant, you're good to go. "DOT APPROVED" doesn't mean anything as the DOT does not set lighting standards for the US automotive lighting standards.

Since thay are listed as they are "FMVSS 108" compliant, you are good to go.