Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Halifax to Rapid City as you normally would.
When travelling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Halifax is roughly greater than that of Rapid City. You can assume this to be the case over the next 17 days.
You can imagine temperatures to be in the 33 °C range in July. The weather in Rapid City is generally hotter than Halifax's. As flight availability is subject to become more limited as peak season approaches it is ideal to book in a timely manner.