Hi there,
1. Don't worry too much about mileage. The thing you want is a bike that's been looked after and serviced regularly; then you can be confident even if it's got 30k miles on it.
2. The ones you've picked out look like great choices. Honda and Yamaha make probably the most reliable machines on the road, and they handle the salt in winter better than others do too. Being scoots, you get a load of protection from the weather, which is nice in winter.
3. A topbox is a great accessory, if you're commuting (which I guess is what you're going to be doing)
4. Textile jacket and trousers. Goretex is best, but very expensive. If you want to save a bit, there are often good secondhand buys on ebay.
Go to a shop to get a helmet. Must be new. Anything that you can buy from a reputable shop will have good protection, so the important things are: is it comfortable? does it fit well? and how much does it cost? Get the folk in the shop to advise you about fit, because it needs to be tighter than you'd think. Personally, I'd get a full face (or a flip up) if you want to ride in winter. Then boots and gloves.
5. Ride smoothly in the wet, especially around roundabouts (where diesel spills are more likely).