Since your 18 and I am 70 I'll give you some thoughts from a old person. I am not from Colorado but I understand OC is legal there in some places. Please correct me if I am wrong, but to open carry you have to be able to own a gun legally. I think you have to be 21 as you mention.
So getting a gun while under age, and then open carrying, how wrong can that be?
Now, about the G17. I love mine, works as advertized. Reliable with any ammo, has never failed to fire.
The rest of your questions can be answered by your local gun store.