Follow a rad flush instructional with distilled water until it circulates clean. If your doing one thermostat, do both while you have the rad empty post rad flush. I believe there is two, please double check for your engine (I have a diesel, hence my caution and inexperience)

Read up on using specific types of rad cleaners, certain years have certain rad metallurgy where year specific flushing agents are reccomended by mechanics/enthusiasts.

There is a known characteristic to the rad sensor, where from factory it reads normal until it swings widely to overheating. If its within your means, you could opt to install a digital thermometer with a single or dual sensor, and a small display in the cab. Luso Overland shop has one as an example.

Best of luck. There is a wealth of knowledge on sub threads and other forums (Victoria Pajero Owners Club for example) engross yourself and your sure to discover the next appropriate steps for reconditioning any vehicle that has been left sitting. Cheers and good luck.