7.62x39 is more than good on deer, out to about 150yds. I used mil surplus w/open sights for yrs. Dress for the weather and then some. If its cool you will know it after sitting trying to stay as motionless as you can for hrs at a time....... no standing up and doing jumping jacks cause you can't feel your toes or limbs. Listen to your grand-father he more than likely knows the land and how to hunt it.
Be true to yourself, know your limitations and make sure your grandfather knows too. That way you won't be asked to shoot farther or do something that your not ready for.
Have fun and get lots of pics...... I can assure you this will be a hunt you will treasure for a life time.