A 25 merc 4stroke weighs 157lbs[lighest version] if u can get your hands on a 25hp 2stroke at 105lbs get that and it will fly.A guy i know has a new 15hp 4stroke merc which is a 15hp tohatsu same motor and it is great and dependable motor one of the best made on the same boat and with 2 people he gets around 25mph which is good but it takes a while to plane because of weight at 114lbs,as with a 15 hp 2 stroke they usually weigh in the 77 to 79lb class and good low end torque u will get up on plane fast.
The only motor i would suggest in a 15 at the moment is a 15hp suzuki,after doing extensive research that seems to be the best 15 4stroke because of weight and money.At just 97 lbs and great low end torque u can't beat it for $2300.U never have to change a oil filter ever so there is a plus and it is lighter then any 15hp 4stroke period.The best motor period is probally the honda at 101lbs but now your at $2700 to $2900,also a oil filter to change.Don't be afraid to buy a suzuki they are 100% dependable and i think they are way ahead of eveyone else in boat motors as far as technology.Good luck