Sammi, I would say that you have yourself a situation! There are two BIG holes in your transom. So, you gotta rebuild the transome to go with a single. What about the stringer setup? If you go with twin 240hp Yanmars or volvos, its gotta be $50K+easy. If you go with a big single (and asociated big drive), it will be close to the same number with the transom work stringers etc. You can get NEW 5.0 MPI 's WITH the bravo 1 drive for about 13K a copy. So you are 30 K with new injected gassers (2) 20-30 G buys a lot of Fuel! And those 5.0 MPIs are thrifty with gas.
check it out at
Carbureted Sterndrive Mercruiser Marine Engines at Performance Product Technologies
Diesel is cool, but pricey. I am no mathemeticion, but $30k would be hard to make up in fuel savings I think.
Now, If you are like Jason (LOADED WITH DINERO) go with the volvo
Global - Volvo Penta : D6-310/DP