A temp gauge requires a sensor - what you have now is a 'switch'
You ARE in luck because Beta Marine does offer one.
Beta Marine | Marine Diesel Propulsion Engines and Generating Sets
The sensor and a guage will likely set you back about a 150 bucks - and they are not hard at all to put in.
Coolant flush should only be needed about every five years. When I was in the Navy they did a study and found that the most common cause of failure on a lot of gear was the preventative maintenance (i.e. if it works, leave it alone).
I'd install a gauge (I am in the process of doing that on my new to me boat), fix the leak, fill the system and leave it be unless it overheats.
Also if you don't have one, get a book on diesel maintenance and troubleshooting (Calder's is best I think); or take a couple evenings and sign up for a class at your local community college, some have excellent programs.
Folks don't need to know much about cars these days, they can pull over and call a tow truck - hard to do that on a lee shore at 4am when the rode