Considering you have no interest in gaming or other compute-intensive tasks, I recommend saving your money by purchasing the Celeron system. I perform the same tasks as you with my notebook, and I am still using a Pentium 3 with no performance complaints.
The service plan is a personal choice question. I do not purchase such agreements, but I also service my own equipment.
If the notebook is going to be abused or used daily, then a service plan may prove beneficial. Notebook replacement parts can be quite expensive, especially if something happens to the system's screen or motherboard for whatever reason.
Robert Richmond