The melting temperature depends on the type of plastic used in the mold. Many of the common molds (either single cavity or smaller flat sheet molds) will melt starting at 135 degrees F.
Some plastic log molds are HDPE plastic which have working temperatures of up to 180 degrees F. Other "lighter duty" plastic log molds are only usable at much lower temperatures.
Pour temperatures for MP depend on the brand of base, but you often see 110-120 degrees F as a pour temperature.
It's definitely worth getting an infrared thermometer or a glass candy thermometer. You can get an infrared thermometer off amazon for under $20 (excluding shipping).