When I started with the incursion stuff, I had help from expirienced multiboxer incursion runners. They spoke of a safe tank of 3500dps when planning fleets/fits/doctrines. Thats a very safe number, but safe is safe.

Now, for my fleet (8 Rattlesnakes, 2 Nestors, 1 Vindi, 1 Astarte) its the same like ToS, I mostly use 3 reppers from a single nestor, sometimes I need to put 4-5 on if I m late.

What I would advise, is start your marauders with a strong tank, learn the kill order and get used to the sites, and then graduatly refitt to more damage and less tank. This gives u plenty tank to make mistakes and you can look forward to up the damage when you start feeling the itch for more dps. (The itch usually happens when u start getting comfortable with the site and the killing isnt fast enough anymore )

My 2 cents