What size is the merc? Just to give you an idea the msrp for a new mercury 40 hp 4 stroke is around $5k. Not cheap for most people. Some of the larger ones in the 200 hp range can cost over $20k! That's a car lol.
If you already have a motor then you should be able to find a decent boat for less than 1500 without much trouble.
Personally I have a 115 hp merc on an 18 foot Lund deep v fishing boat and I love it. Fully loaded it's not very fast though. Probably maxes out at 40 mph on a calm day. the op could probably hit 55-60 easy wot with his boat. I plan on upgrading to a 24 foot class center console with twin outboards whenever finances allow. With student loans coming in it's not looking too good for a few years though

. I want something more seaworthy than what I have now though for some inshore fishing.