I think an F2 is a wonderful generation to have. You get many of the serval traits without as much of the persistence and stubbornness that you may find in an F1. I don't believe you need to have an outdoor enclosure, but you definitely need to Savannah-proof your home - for any generation really (don't worry, if you miss something they will be sure to point it out to you

).
As Brigitte has explained, Savannahs come in all sizes. I have had four F1s over the years with the smallest being the size of a fairly large domestic cat (but probably not as large as a Maine Coon) and one who can kiss my chin if she stands up on her hind legs on me (I am 5'2").
As far as being home alone, well this is where a companion is so important. I work up to 12 hours a day, and when I come home my cats let me know I've been missed, but after the initial brief greeting they are much more interested in getting dinner

However I have several Savannahs and they are very good at keeping each other entertained during the day. If I had only one cat I'm sure that would be a very different story.
Savannahs are very intelligent cats so training them is not difficult as long as you provide them with adequate motivation. As Brigitte has pointed out, they are much more adept at training
us than we are at training them! And just realize, if you try to train them to do something that they simply don't want to do, you will likely never get them to do it...
Regarding introducing two kittens, there are many threads on this forum about introducing new kittens to other pet family members so you can do some research here. If you choose to get two kittens from the same breeder I would still try to isolate them from each other for at least a couple of weeks so that they have the opportunity to bond with you, which would be less likely to happen if they already have each other. If you are introducing new kittens to each other I would definitely give them a two week isolation before introductions - by then they will have learned that there is another presence in the home and have gotten used to their scent (on you) which will make introductions easier. Each cat/kitten is individual so there is no cookie cutter schematic for introductions, but as long as you take it slow and easy, and be sensitive to the kittens' reactions and adapt your plan as needed, they should do fine. BTW, if you choose to go with two kittens, they don't both have to be Savannahs. Any high energy breed should do well with a Savannah. I have both Savannahs and Highlanders, and they get along wonderfully.
As far as ownership experience goes, if you have lived with other high energy, intense cats then you should be well prepared. If not, then it will be a learning curve for you as well as your new Savannah. Just remember that you never want to leave anything out that you cherish, and everything is a potential toy to a Savannah, including power cords, drapes, wall paintings/hangings, throw pillows, cabinet doors, external refrigerator ice machine, etc.