Are you buying the rig new?
What size sportfish?
The 115 will get you by just fine, but it wont be a speed daemon (1850 is rated for 175hp). I have a 115 Yami 4 strk on my 1850 Sportfish and consistently do 38 mph (gps) with a full tank and a couple passengers, but have never seen 39mph. I am able to pull up skiers without much of an issue, and that is with a full tank and 4 guys in the boat. A 150 would be nice and I would like to upgrade at some point, but it is not required. It would be nice to have that additional speed, holeshot, and run less RPM’s at higher speeds.
I would strongly recommend getting a SS prop, and taking the time to test a few to get the right one if need be. that will help with the lower HP engine.