6-7 weight to start for sure. I tried to learn with a 4wt and was frustrated for 3 years and gave up for 2 years. I got a 7wt and learned how to use it very quickly and using the 4wt was easier to adjust to.
One thing I have read on fishing from a boat is that if you are not standing up you mgiht want a longer rod. 11' seems to be what was rdecommended just to keep the backcast off the water and get a good load for the cast. I do okay in my pontoon boat with my 9' 7wt but I do sacrifice about 20-30% of my distance.
As for all the different types of fly line, start with weight forward and then learn to throw the others. They do cast differently so trying to learn with all of them will give you a head ache and maybe a couple flies in the back of the head.
I love taking my kids fishing, now if I could just manage to fish at the same time.