1. Start with a free site, if you are just entering this game. However be aware that one free site won't make you much money, you need a hell of a lot of them.
2. Yes, but with scripts there is free and there is good, there is rarely both in one package. I always advise people to learn Perl or PhP and write their own. (If you intend to make a living in this business and you want to buy good scripts, you are going to spend several thousand dollars on this. A good book on Perl will cost a fraction of that.)
3. Niche is best, but do not pick a niche that you think is the best money maker. Go for the niche that turns you on, you have to know your market to make a lot of money.
4. Yes and no, with a definite maybe. Simply put, you will improve your SE ranking by increasing the links to your other sites. But if your surfers get too many links they are going to be satisfied with all the free porn you are giving them and not buy anything.
5. Don't go down that route in the beginning unless you have money as start up capital. People don't post to LL or TGP in a reasonable quantity until you get a lot of surfers, and you won't get a lot of surfers until you get a reasonable quantity of posters. So to start with you will be buying in traffic, and as you need thousands of hits every day this is expensive.
However if you do want to go with TGP take a look at these tools they might help:
Free gallery submission script (hosted)
Gallery Pool (where webmasters can post their galleries and you can add them to your site as starter galleries)
Another gallery pool