As l was driving downhill the temperature lowered to about 60 deg Celcius (without thermostat) and then when uphill the temperature raise to 80~85 deg C. Is this okey or running at very low temperature could affect the engine is life?

depends on your location. no thermostat is fine in summer. winter you need it for heat and defrost to work properly

just a opinion here , Usually the thermostat helps bring the temperature of the motor up when its cold , If the motor is changing temperature esp getting warmer under load & cooler down hill its most probably a radiator problem .it may be low on coolant but most probably the grills r blocked with leaves & the air is not flowing though . A lot of newer cars have like twin radiators & u cannot see between them due to a rubber strip the leave s get in there . This problem happened to my ford,

Engines now days are designed to run in a +/ - 10 degree range.
It is not recommended to run them without the thermostat installed. You use to in the old school days But not now.

Good Luck

Its true what NEWBEE said older cars it did not matter about not having a thermostat but not anymore the computers in newer cars need the engine temp to stay consistent for the purpose of timing fuel delivery air mixture and a load of other things. So with a newer car you can not go messing with things.

The thermostat is a very essential part in the cooling system and its not good to operate without it. If the thermostat is not there, then the engine would not warm up quickly which would result in poor efficiency as the engine needs to have a minimum temp. for optimum performance. It is not adviceable to run the engine without the thermostat valve.