Take what I am saying with a grain of salt since it is just a regurgitation of what i have learned but it has worked for me. You never want to give customers a break down of parts and labor and especially not quote labor times. What you have to do is talk to them about the value of the service, what you will be providing them, your warranty, your training and certs and and the end of all that once u have built a value based service in their mind then you give them the cost at the end. Use softening words like, "Your investment for a professional maintenance service will be XXX." If you talk about price right off the bat without building value into your service, all customers are going to do is think "price price price."
In regards to the spark plug issue I would mention that because you have extensive experience with this engine (make it sound like you are an expert with this engine) there is a possibility you may have to perform additional work. Assure the customer you will do your very best to avoid incurring extra costs during the procedure however you are full equipped and capable of handling the proper repair. Maybe this would be a great time to mention your warranty and that you full guarantee the repair once it is made.