If the motor and Trans is in good shape its a solid boat. They were fairly well made and will last, if taken care of.. Its rough riding to just cruise around on. Doesn't have much of a keel for smoothness
Correct on the first 2
Item 2 for Buzzer
Item 24 for gauge
Item 15 for is the TKS temp switch, which is part of the TKS carb control
https://www.mercruiserparts.com/bam/subassembly/30908/13603/150
Also looks like you need a thermostat housing
Not an OB guy, but seeing a bit of fuel on the water would not be surprising with a motor not running right. If its not burning all the fuel, as in flooding
Look in to the carb, see if the fuel pump is working fuel pump
WOuld like to know if it changes any during the cycle. If the 5V circuit goes below 5V then it can throw all kinds of problems within the ECM. One bad sensor drawing to much current can throw things off