Controlling 12V 70A Motor(s) with Arduino MEGA (or NANO) and a 4-terminal automotive relay
The relay information given indicates that is is intended to control a blower motor. A relay like that, as used in a car, may have its own fuse. If you can identify the blower motor fuse in one of the vehicles that the relay can be used in, that would tell you the maximum current required to drive the relay.

You might find a seller that sells it for general use. Such a seller may give more complete specifications rather than a list of compatible vehicle applications. Some sellers show a photos of their products that have sufficient detail to read the manufacturer's information printed on the product. From that you may be able to find more complete specifications.

A relay that is intended for starting and stopping a dc motor should be able to withstand the arcing. The motor should be able to withstand full-voltage starting if it is a motor that is sold for a use in which that is the normal starting mode. If full-voltage starting in your application produces an unacceptable jerk, you could make a reduced-voltage starting circuit using one or two series resistors that are shorted out as the motor accelerates. That may or may not be less trouble and expense than using an electronic speed controller.