If your going to put the turbo diesel in it, your definately going to need a Bravo Drive to atleast somewhat handle the torque better. Judging by the initial 4 cyl merc, I'm guessing your running an alpha right now?
If you want to buy a brand new complete drive for cheap, and I mean dirt cheap http://www.sterndrive.cc/
They make an aftermarket Alpha drive. We have one on my chicks boat right now, and honestly other then alot of u-joint noise for some inexplicable reason, it runs perfectly.
If you want to save some money on a new Bravo 1, There's a guy selling one in the spam section of hotboat for 4K (brand new drive 0 hours) you still have to buy a gimble, trim pump and a prop though.