Sacha, I think your hair looks great.
As you say, it looks in the pictures like you've got a lot of natural hair.
I think you could get away with quite a high density like 80%.
Don't worry about the different dryness/texture of hair because a little wax/gel or Brylcreme will even out the look.

As for the grey, a lot of people naturally go grey only at the sides and back so you wouldn't necessarily have to have grey added to the replacement hair.
I think the choice would be yours.
But by sending in a decent hair sample, you will get a good match anyway.
You also might want to think about what shape hairline you will want.
This is called the front contour.
At your age, you could have a very straight hairline.

The 2 important things to remember when ordering a custom hair piece is to send a good hair sample and make a good template of the exact shape you want and details about the direction you want the hair to go in.
With your current style I would suggest 'forward ventilation' from the crown.
Sending in these pictures with your order is also a good idea.
I would also suggest ordering a body wave curl because that will give your hair 'movement' when you style it like this.

Good luck.

PS. I've never heard of Super Million Hair.
I presume this is a concealer/thickener?
Is it a spray or powder?