Question 1, yes you will have to do maintenance. Change the oil, spark plugs and so on when they need it.
Question 2, yes you can play in the saltwater just as long as you flush the motor with fresh water afterwards and then wash everything with soap and fresh water.
Motor flush is very easy to do. You turn the engine on, and then hook up the hose to the flush kit and then turn the hose on. Let it run with the hose flushing the ski out for about 15 minutes. I also like to rev my motor every few minutes for a second or two. Then, turn the hose off, turn the engine off and disconnect the hose.
I can answer for your number 3 question. I just bought one around three weeks ago, and gave 7990 after taxes, setup fees, crate fee, title, etc. So 7990 OUT THE DOOR. It is a GREAT ski, and I haven't regretted it one bit. Good luck!