The problem with your idea is that means a lot of time spent in the Van Allen belts. Not a good thing for a healthy crew. You want to traverse them as quickly as possible.
The Van Allen belts aren't that big of a problem; shielding against them isn't overly hard, and if you're using an ion engine, you can run a course that partially bypasses them anyway.
You could also provide for both, going light and fast on early missions until you can launch crudely processed lunar material for radiation shielding, and then add that to your ships along with the ion drives. The internal core of the ship (the expensive part) thus gets re-used.
I'm thinking the crossing through the Van Allen belts wouldn't take so long that the crew couldn't ride out that part of the trip in a heavily-shielded central node (three or four days through either belt, depending on the flight profile). Heavier shielding would be an upgrade with better technology, but again, we're talking 2020s, 2030s timeframe and that might be available by then.
OTOH, I've heard Franklin Chiang Diaz suggest the VASIMR's magnets could be used to provide shielding for a craft during CMEs. A similar technique may be applicable in this case (also, using a VASIMR could reduce your transit time from three or four months to three or four weeks