I own a 1979 SeaVette that was originally powered with twin 200 hp Evinrudes, mounted on the transom. I had the transom replaced and mounted the O/B's 30" back on a custom bracket. Second smartest thing I ever did. First one was having a professional do the job.
You do not mention what the current powerplant is. I assume it is an I/O. As to your decision, there are pros and cons to both setups. The I/O engine is easy to work on, fuel efficient, and very reliable. If you currently have an I/O, it should be relatively easy to replace the existing components with new ones. The gas tank is designed for an I/O, and your center of gravity will remain the same. In other words- you will not be surprised the first time you take her out. Outdrives have come a long way and many of the problems that plauged them in the past have been corrected. However, they still sit in the water and are subject to all the problems of submersion i.e. corrosion. You do not mention which manufacturer you are looking at. I would recommend Volvo. I own a '69 Donzi with the original outdrive, it has never been rebuilt and I am pushing 300 hp through it. Not one problem ever. I would never own another brand of outdrive.
The outboard is also a good choice. Moving the outboard back 25 to 30 inches will increase the responsiveness to trimming a great deal. Mine was like night and day. Outboards have also come a long way. They are relatively fuel efficient, reliable, and you can tilt it fully out of the water. The downside is that they are very expensive to work on. If you are converting from an I/O, your gas tank might not be large enough to give you the cruising range you would like. Your center of gravity might change also so you may be riding lower in the bow than you used to, no matter how much you try to trim. It may sound trivial, but your towing characteristics will change also. Pushing the motor back that far will give you a lighter tongue weight, requiring adjustments to your trailer. Mine required moving the axles back.
My GPS shows my boat to run at 56 MPH, at 5400 RPM, and I chew about 40 GPH WOT. I have a 126 Gal gas tank.
My recommendation is to stick with what you have. If you have an O/B, you would be wise to redo the transom and add a bracket. Make sure the bracket adds flotation as well. Get the longshaft version of the motor!!
I apologize for the length of this response but it seems you are dumping a lot of money and effort into this project and you want the outcome to meet your expectations. There is no one right answer, you must consider all the factors that are involved. Time and money are the biggest. If you wish to discuss this more I would be happy to. My e-mail is
tunsmoor@comcast.net. Good luck!