If you are worried you'll stutter to your doctor, write it down. I write stuff down for my doctor all the time. They are used to that.

For the shop guy, just ask if he can do a fit check and check your posture. Let him know you are interested in fit. Tell him you are getting a custom bike in the future, but you want to make this one fit as best you can now. Expect to pay for a full fit session (which is about an hour), but a quick fit and posture check (about 10 minutes) is not likely to have a charge. Give the guy a plate of cookies as a thank you, and give the shop your business.