Diesel vs 4 stroke outboard???
Not sure there is a correct answer. Having owned boats with both, in general, I would take the 4 stroke OB over the diesel. New outboards are VERY reliable and low maintenance. The 4 stroke Yammy is fairly fuel efficient, especially at 9.9.
However, it seems that you wish to make a very very long trip in a small boat that might require lengthy periods of motoring. In general diesels are the choice in this case. The price you mentioned makes me think you are thinking of a used diesel...
SO, my advice would be to stick with what you have and put the money towards gas for the trip. A used diesel can be very problematic. You have the up front costs and unknown costs should it break down in the middle of your trip. In addition, not knowing the boat, the above comments the inboards suitability for your boat my be significant. If ANYTHING happens to your OB during your trip, you can disconect it from you transom in seconds, lift it up yourself and carry it to a dealer (or the dumperster and get a new one for $2000).
Keep the outboard in the mid portion of its rpm range and you will be fine. Save the cash.