Ciao Flyc,
Glad you found our forum and are finding it useful!!
Yes, foreigners can buy tickets online at the Trenitalia website but I believe tickets are only on sale for the following 2 months, so for December you'll need to wait until October. You actually don't really need to buy tickets ahead of time unless you have special seating you want but you decide, I know sometimes one can feel more reassured having everything on hand before and not having to go get the tickets once in Milan would be one less thing to worry about.
You have a great itinerary already planned out - your hotel is in a very central location and most definitely within walking location of all of your planned sights/museums.
As regards the 17th and visiting the outlets: The Mall and Fendi are basically in the same location, D&G is not that far from there but Space is definitely in a different town and further away. Have you seen this article on the
outlets in Tuscany and how to get to each of them?
I don't think a taxi is the best solution for your day unless you were to establish a cost for the entire day... that is hard to do ahead of time. So I would recommend the service of renting a car with a driver so that you have a set price from the very start for a full day of shopping at the outlets. If you search for "florence car with driver", you'll see that there are many companies offering this service. I can only recommend this one:
http://www.enjoyflorence.com/italian/outlet.html and, as you can see, they offer the same itinerary you're planning. Get in touch with them and see what they propose. Since I haven't actually tried these types of services, it is hard for me to tell you which one to choose. You'll have to see several and ask price estimates and decide which one you like the most.
Hope I've been helpful!! do let us know if you have any other doubts or questions!!
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