How easy do they tame?
Very easy, when you get everything correct and give them time to acclimate. Acclimation for WC seems to take about a year. CB pretty much right away.
What is best to use for an indoor winter enclosure for flooring?
I use a 50/50 mix of sand and top soil or dry bed-a-beast. It gives them good footing and most Greeks like to dig themselves into the top inch or so of the substrate if you don't provide a hide box. If you buy a hatchling or young tort MAKE SURE you have a hide box and keep the humidity in the box at 65-80%. It will help keep your little one from pyramiding, given you feed it the right stuff.
Do they eat pretty good?
Absolutely, again once they are acclimated. I've been feeding ours fro mthis list for a long time. Chopped cactus pads, garden weeds (dandelions greens and flowers, mallow, plantain), hibiscus leaves and flowers, sedum, wild mustard, and grape leaves. When they are inside it's collard and mustard greens, endive, cactus pads, and other dark greens. I supplement with calcium carbonate and have used Wallkabout Farms grassland tort mix for a few years and it works great. Never, ever feed any type of high protein product. No dog or cat food EVER! No fruit (except cactus pears, and only one in a while) and no vegetables (except pumpkin once or twice in the fall). It's a natural dewormer that works well if you keep your outside during the summer and early fall.
Couple of thoughts. Buy a captive bred Greek, it will save you time and money. Most CB's are Ibera's so you'll be getting a tort as hardy as a Russian. If you have a garden to plant purchase mixes from either TurtleCafe.com or TurtleStuff.com. They both have excellent weed and flower mixes that I have used in our outdoor pens with great success.
Good luck!