If you do one mission at a time, that is what I do, and you do the epic arch/storyline missions manually, you will never need more than 8km3 cargo space. Currently I am flying a bestower that I have taken almost all the cargo extenders/rigs out of. Now it has Hyperspatial Velocity Optimizer IIx3, Nanofiber Internal Structure IIx3, and Expanded Cargohold IIx1. The bestower now has something like 8.2km3 and warps at 7.4AU rather than the 4.5AU it started with. The non warp speed has gone up also, I THINK its 250 now rather than 125 m/sec it starts with. I do not know the align speed.
So far with my looking the Viator seems one of the better ships, with 9AU, 3K m3 cargo, both un modded. Not factoring in the Gallente Industrial Skill Bonus, or Fit. However, that ship is almost 30 days away for me
Viator