Fuel and rpms are the 2 most important guages and a temp guage for the motor would be adviseable as well. Always good to know what the normal temp is for your engine since if it changes form the norm, you will know something is up. Heat is a major enemy with 2 stroke Outboards. Good news is your engine is likely to have an audible over heat alarm so it if gets too hot it will sound.
I would fix the auto bildge switch. It's a cheap safeguard if you start taking on water and you don't know or are not there to turn the switch on manually. Make sure the pump and switch are direct wired to your battery so they have juice to them even if your ignition or main battery switch are off.
If you are not going to keep it in a slip no need to waste money on bottom paint and a transom mount transducer should be fine. You just need to be sure to mount it where it will not get hit by your trailer bunks or rollers.
Best way to monitor fuel consumption is a flow scan. Some GPS units have a kit you can buy and install in line that interfaces to your GPS but given the size of your fuel tank and how you plan to use the boat, I don't think a flow scan will have a lot of benefit for you compared to the cost.
I would also plan to swap out any plastic deck hardware with stainless and if you have any thru hull fittings under the water line which are plastic, switch those out to medal as well and make sure any hoses below your water line are double clamped.
Lastly, if you don't have one already, invest in a fixed mount VHF radio and a decent antenna. It's a good insurance policy and I would not leave the dock with out one. Cell phones are great but if you get in any real trouble not like you can just get on 16 and broadcast an emergency help message. Besides that, the Coast Guards and most towing companies can find you quicker if you are in trouble since most have VHF radio direction finders to help them get to your positon quicker.
Best of luck with the new boat and if you are new to boating you might want to read this
http://www.njfishing.com/NJ_Fishing_Safety.php
