to answer your questions. 1 braided line works great, I use 20lb fireline. the leaders are mono anywhere from 14-20. mono stretches so when the fish bite, it'll give a little and yes that leader goes between the dipsy and lure.
2. I troll floaters and shallow divers behind the dipsy's and jet divers. deep divers pull too hard and don't work right. troll suspended baits too, you can pull em even slower if you need to. I use rapalas, rattlin rogues, bomber's, thundersticks and reef runners. its key to make sure that they are running right so drag the bait along side the boat for a sec and make sure it looks alright.
3. when you do mark fish you want your lure to be over the fish. Not eye level or underneath. The fish will tell you how far they will come up. If your markin fish at 40ft in 42 ft of water, your baits should run at or around 35-38 ft. the better you get at depth control the better.

Let me know if this helps you. Its just a guide to point you in the right direction.