I was diagnosed with Crohn's Disease when I was five, i've had one, though I didn't really have a choice. I don't remember much about my hospital stay but going for a colonoscopy is one thing I do remember.
I personally don't think you should have one if you don't need one. You are going for a general check up, you don't seem to have any serious illness. You probably wont gain anything by having one but the likes of me, who does suffer from a disease, would. I don't see why someone should just have one because they feel like it when there are people out there who actually, really need to have this done.
I don't know how you go about getting one, I didn't have a choice, I was given one because I was actually extremely unwell. I don't think your doctor will just give you one out of the blue if you are showing no symptoms of having any problems. Just ask and see what your doctor says.