I’m ready to start but I don’t want to spend unneccesary hours. Is the easiest way just to trace a blueprint and connect the vertices or should I use a box and work it from there?
Yea you want to find some blue prints just so you can see the shape of it from different perspectives. However you should also get A LOT of reference images because there is a lot of detail in a tank that blueprints just don’t show that well. The way I made my tank was to first start of with the treads, just making the wheel part then duplicating it to get the rest of the wheels. Than to make the actual tread all you have to do is just make on section of it, then duplicate away and to place them around the wheels. Than make the armour thats above the wheel (just get the basic shape of it, add detail later) then make half the body (not the top part that holds the gun) and just duplicate everything and you have the entire base of the tank, then add the top part and gun. Here is some images of my tank of a Jagdtiger in case your interested.
To me it would be easier to start off with a plane and extrude the edges and then extrude later but its up to you, just make sure that you have wire mode when you trace (press z)
That’s difficult to answer, actually. It depends on your modelling style - and that is something, that you will need to find for yourself. I’d actually say that this is a perfect project to try both ways and see which one works better for you.
No, that’s not lost time, to know which way you like best will be a great help for your projects in the future. Blending is NEVER lost time 
I personally am a Box Modeller. However, sometimes it’s better to trace something and then extrude. Heh, that’s really more difficult to answer than I thought…
wow. I love the treads. How long did they take?
I think I spent about 1-2 hours on them, didnt take as long as I thought it would.