Let me point you in the direction of the Charon and the Obelisk.
The Charon is likely the most efficient @ Freighter IV you get 942,000m3 of space and @ V you get 981,250m3. This is significantly bigger than the other races and gives the most bang for your buck.
However, the Obelisk is likely the most common freighter due to the Gallente Industrial V and the fact this is a large chunk of the prerequisits. an Obelisk is 937,500m3 at level V.
The Obelisk is also much tougher 200,000 ehp as opposed to the 180,000 of the Charon.
Also if you are planning on ever extending to Jump capable freighters the same relationships hold true.
EDIT: to take into account your changes.
Ok the Provi has a nice tank, personally I think it's the best looking freighter also. If you are after efficiency, and skill training isn't going to play a big role, you pretty much are ruling out all the ships except the Charon.
As CanHasGank has stated, top speed don't mean very much at the end of the day, you will be going gate to gate. I don't think your 70 m/s is accurate, my charon does nearly 90 m/s you sure you have navigation V on? Evasive Maneuvoring V and Spaceship command are essential skills if they aren't obligatory (can't remember) you should get them anyway.
To answer your tank question, tank in a freighter is basically irrelavant, a Freighter has so much EHP that as long as you are going gate to gate (and not autopiloting) no chancers will bother you.
And unfortunately if there is an organised hi-sec gank eyeing you up no amount of tank will save you.
If they point you, they will be webbing you too, you won't be able to crash back through the gate, you won't be able to warp off, all you can do is hope they have in sufficient DPS and concord will deal with them.
Lastly a dual Webbing alt is not a nescessity in hi-sec, in low or null sec it pretty much is what keeps you alive, but in hi-sec it's nothing more than a timesaving convenience as it pretty much saves the need for aligning.
Oh btw, don't forget to make sure your webbing alt is in the same corp and fleet as the freighter pilot or he will get concorded.