Yanmar 6LPA 315

What I can expect from well maintained Yanmar 6LPA, 5000 running hours?

 

Clean oil and run at 80% of hp, 5-10,000 hours is common. Some go twice that. The problems are usually in the accessories - pumps, heat exchanger, etc.

 

What is the propose of marked part?
Imgur https://imgur.com/a/8pCQNvT
Imgur https://imgur.com/a/RpkliTs

 

You'll have to trace the wiring. One guess is a relay. It's become common for engine manufacturers to use a relay for the start circuit. When you turn the key or push a start button the power goes from the terminal on the starter where the battery cable connects - to the key - and back to the start terminal, often marked with an S. That can be a long run and with 12 volts, you get a voltage drop. If there's too much of a drop you'll eventually burn out the solenoid. With a relay the power travels from the cable connection - to the relay - and to the S terminal. A very short run. All the key does is supply a small amount of power to the relay coil, so a voltage drop doesn't matter. It also protects the key contacts from burning out.