Hi Westy,
That's a very doable style for a full cap. I often hear from hair wearers that have made the transition from topper to full cap that the full cap has it's advantages. You don't have to worry about matching the color with your own hair. And you don't have to worry about blending the cut with your own hair. I think longer hair is always easier than short hair because with you have more leeway. 40% looks about right. Super Fine Swiss lace is a good choice. With that density you could also go with Ultra Lace. The downsides aren't much--just use a broad tooth comb when you first wake up so that you aren't pulling too much on the hair--then you can go to a normal tooth comb. With a full cap it's always a good idea to go with poly on the back and sides since you have the movement of you neck and you want to get a strong secure hold. You're very realistic about the look--it's age appropriate. Many guys want to turn back the clock and go with a density that's too heavy.
That's a great style for a guy your age.
Regards,
John