I use a 70 qt Coleman xtreme for 5 gallon batches with no problems. These coolers are insulated very well and I always maintain my temperature during a 60 minute mash. I do preheat it though. My only complaint is how much room it takes up.
I think either one will suit your needs for 5 gallon batches just fine. But with that said, your needs may change in the future and you might decide that you want to start doing 10 gallon batches. If you can afford it I would go with the 15 so you arn't restricted in the future if you decide to...
Pretty much exactly how I do it. I like to let the keg sit upside down with oxyclean for a while to get inside the posts. Every so often I will take all the kegs apart and clean everything real good.
I've been using a wooden dowel with no seal or finish for quite some time now, with no issues. I only use it pre boil or just before flame out. Though I have been meaning to etch to the old dip tube from my keggle.
Very impressive. I was seriously doubting 4 kegs fit in that freezer until I saw the pic. I wonder how precise the tempature can be controlled between the two, but either way, very cool.