I would not go below 140hp. Although speed is not the biggest issue, 115hp will have a hard time keeping that boat on plane in a 4 ft chop, when you need to slow down but stay on plane this small engine will fall flat on its face and you will regret going that small. For typical use it will work, cruise slow, but be efficient which is ideal for some, but it does not have the torque of small diesel inboards that are common to go slow with small hp in boats this size, they have no problems staying on plane at low speeds. no price is a good one for that power on this boat persoanally. I would go with a 140 suzuki as min power, it is very efficient and would push the boat well. An F150 is a good package too, or look for a left over HPDI, they are great on fuel and can be found cheap if you can still find one new, and they are great on 20ft hulls. I do not believe Grady put anything below a 150 on a 20, yes they are looking for a set speed, but they also want an engine package that will give you performance that is good. A Grady hull is not designed to cruise in the low to mid teens, it will not handle seas well and will be very inefficient with smaller power. You can get awya with going maybe 10 hp each way, but a 35hp drop is a bit, your talking 24% less power then the min Grady installed, that is significant.