random thoughts
Weather/When to do it
We have done the ride in June and September. Both avoid the busiest holiday season, but should offer reasonably warm weather. On the June trip we had one very wet and quite cold day and there was some rain the next morning. Most of the time the temperature was very pleasant, although by the time we got to Spain it was getting a bit hot. On the September trip we had the odd shower, but nothing too bad. The temperature was generally just about perfect. I am sure you could make the trip in colder wetter weather, but it would be far less enjoyable.
What to take
Cycle clothing - the clothes you ride in will want washing every day. It is probably best to have 3 sets so that if you can't/dont want to wash/dry your kit for a couple of nights you still have a spare. Some hotels will do laundry, but not all. Have some handwash liquid for clothes.
Spares/tools - things do go wrong with bikes so if you have the luxury of a support vehicle taking some spares can make sense. Beyond the obligitory inner tubes, a spare tyre or 2 and tools to make basic adjustments would be first on my list. Gear changes and in particular front deraillers seem to be the most common issue. Some bike cleaning gear can be useful in case you get bad weather and they get clogged up with dirt.