The 2.5 hp would work fine in most conditions. I once had to push a 25 foot inboard for about 10 miles with a 2.5 hp dinghy motor (on a transom bracket)--that boat had more drag than the C Dory 25 (fixed prop, heavier, keel etc).
As I have noted in the past, I lost the transmission on a 62 foot motor sailor, 65,000 lbs. I was able to push that boat at over 3 knots with a 4.5 hp outboard on a dinghy--with a 25 hp (after I inflated the inflatable) I got the boat up to 5 knots. I have also logged thousands of miles in a 26 foot sailboat with a 5 hp outboard as its only auxillary--again that boat was bigger than a C Dory 25.
Many people are able to idle down the main engine for trolling. Modern outboards are very reliable. But some form of a kicker is advisable.