First off, don't believe the forecast. You'll get a general feel for the weather closer to the start date, but many things can influence the weather pattern before then. At least I hope so!
Mudguards - for me, definitely. Nothing worse than riding in the wet without them, and I plan to ride with others when speeds align, so for their consideration I would have full length mudguards in place.
GPS charging. I charge mine off a dynamo. Generally I keep it fully charged, then disconnect when it rains. Rain doesn't usually last long enough for the Garmin to completely discharge. If it did, I have a spare Garmin. And if that then became fully discharged, I would be charging the first Garmin in a pannier, using a long USB cable from the Igaro.
Shoe covers - I've not bothered previously, but having seen others wearing them to good effect on wet rides in Wessex, I've now got a pair. If it is warm I'll not use them, but I suspect we'll not be blessed with warm weather at all times. I've used them previously in winter conditions to good effect, although nowadays I use boots throughout the winter.