Temperature gauge and or engine problems

I recently acquired a British import Ford Ranger/Courier freshly serviced and at 270,000 km.

Truck went a good three months while I was overseas with little use and no worries.

Last week it was needing a top up of oil so then I decided to also top up the coolant (above the 'full' mark). Then a few days later the gauge started reading hot, I then pulled over and tried to turn it over again with no luck so I thought it may have been because I over filled the reservoir so I sucked a bit out and cooled the motor before turning it over with the gauge on C and resuming my travels, but the next day it was repeating the same thing. I have checked the motor while it is cool oil cap is coming out clean, I did wipe a bit of dark color out of the radiator and the reservoir is also dark and now it is not turning over.

Big question needing a thorough answer, please help. Thank you in advance.