We owned two prior boats with outboards before moving up to the Olson.
AFAIK you have the basic issue of how much hp is "enough". Our first boat was a trailerable 20 footer with a new 6 hp Johnson which we called the "buzzer" because it made more buzzing noise than thrust (and was amost worthless in reverse).
For most 25 footers, up to around 4 or 5K displ. 10 hp should be plenty. One thing we found was that the 4-stroke engines tended to have more torque at lower rpm (where a displacement hull form needs it/ uses it) than equal-rated 2-strokes.
As they say, "YMMV."
And then, with 4 strokes, a very few are designed for displ. hulls rather than small planning boats.
We had the prop pitch reduced on our first Honda 10, which helped a lot. Then we got a 10 hp "Hi Thrust" Yamaha and were amazed at the quantum leap in usable power. Same half gallon per hour consumption on both engines, BTW.
Mounting the engine is the other big limiting factor. You need a bracket with enough strength, bolted to a portion of the transom with... enough strength.
If in a well, the motor has to physically fit the boat, and the clamping area has to have... enough strength.
Some problems are endemic to outboards in general -- alternators that put out 5 or 10 amps on their best day (!). We found that either of our 10 hp outboards would not quite keep up with normal electrical demands while motoring... instruments, vhf radio, and AP. About every third night we needed to spend at a marina to plug into shorepower and recharge...
In short, for a 25 foot boat, 8 or 10 hp should be enough power, easily.
My .02,
Loren in PDX