I think you should do it. Tulo is turning into an elite SS in his own right -- while he likely won't reach the level of Hanley, the price you'll have him at, as well as the huge boost to this year's title chances, are worth selling the Marlins SS.
My strategy in keeper leagues has always been the same, If you're in position to win it all THIS year, then you do everything in your power to pull it off. As much as we all love Hanley, I think trade will round out your team very nicely and help you vault your way to the championship.
This is definitely a good trade to go for it this year (the Halladay piece being the best part). You still have a good bunch of keepers for the future. Tulo is a decent option at SS and may be a better value than Hanley at that price. It kind of sucks that you would have to decide between McCutchen and Beckham, but I would go Beckham because of position scarcity.