Agree with Ben. A good headtorch is a worthwhile purchase.
As GKD said, start learning what your effort levels are. Race pace runs on the course a great for this, or similar terrain if you are not able to get to the course before the race. Any recce opportunities are worth while too!
You can obviously use the watch early on when everything feels real easy to keep yourself on track but I really wouldn't waste the time and effort trying to charge it on the go. Running is very simple, if you keep it that way you will have a lot of fun over 100 miles
Save the cash you would spend on a watch and other components to make it last a little bit longer and get a decent jacket. I watch is good for training but won't get you to the finish of a race no matter how many wires you have running down your arm, a good jacket might well be the difference. Not to mention the bulk of your training will be in the winter and less than 5 hours at a time.
Gloves - If you are moving well then thin gloves are fine. Bonus of having a crew is you can leave a more substantial pair with them in case it all goes a bit wrong and you are walking it in. Make the most of that and dont carry too much.
Other tips - Practice race pace/race effort running (on the course if possible), practice eating, its a very flat race but hills make you strong so dont neglect them in training and obviously it is good fun so enjoy!
Cheers, I can't imagine doing Ronda. It really is a beast from the small taste of it I got on the little sister event! 30 hours for the winner, a bit too long to be out for me.
I would highly recommend Celestrail though. Not easy, but a fantastic intro to a euro mountain race and Andorra is just a great place to go.
Mitic, I believe, is the most ascent packed of all the races although didnt seem to be as competetive as I expected for a Skyrunner race. Celestrail and Ronda seemed to have closer fields in terms of racing.
The whole weekend is great and its amazingly well organised from start to finish. How they have marked 5 different courses so well over 3 days of racing is brilliant.