Congrats, a nice looking boat. It will shine like new with not much work with the right products.
Thermostat - I bought a 91 19CSS that had sat for a few years, with a 350 mag mercruiser. The thermostat was stuck open and the temp gauge would not go above 120 F while running it on water muffs in the driveway of the previous owner. I replaced the thermostat (you will need a paper gasket to fit your merc) and the temperature moved to 162 F on the gauge. It should be about 160 to 165 F.
Left side hotter than right side- These blocks are known to accumulate sand on the left side. I opened up the left side block water drain (see maintenance manual), I had to insert a wire in order to get water to start coming out of the block. Along with the water was a lot of sand and rusty debris. Every fall for storage preparation, I drain the block and get more sand and debris out of the left side.
Lower Unit - Replacing the rubber water pump is always a good idea, only use mercury parts for this. Using Synthetic gear lube is also a good idea if you plan on running it hard, and protects the gears should water leak inside.
I would also drain the gas, replace the gas filter, put in fresh gas. You can add a carb/injector cleaner additive to the gasoline, such as Techron made by Chevron. Sea foam would be ok, but I don't use it. I use Techron for my fuel injected cars and motorcycle, and it corrects misfiring in no time. Don't use sea foam in anything with a catalytic converter. It will ruin some types of catalytic converter metals. A mechanic told me Sea Foam in late model corvettes results in a $1200.00 bill for replacing catalytic converters.
I would also check or replace the wheel bearings on the trailer. Avoid a disaster on the highway.
