I think you'll be best off with the 24-70mm zoom. You're going to want a smaller aperture than f/1.8 anyway -- at 10 feet, the 85mm set to f/1.8 will give you only a few inches of depth of field. Your example images have a lot more DOF than that. Using the zoom will give you a lot more flexibility with respect to focal length, and also let you change focal length instantly, so you can experiment to get the look that you're after.

One thing you haven't mentioned is lighting. The subjects in your first example are pretty clearly lit by something other than the overcast sky in the background, and I'd guess that's true of the third example as well. Adding lighting to the mix may be more complexity than you're ready for if you're just starting out, but without at least considering lighting you might wonder why your shots don't come out looking the way the examples do even when you match the other aspects of the shot. Take a look at the Strobist web site's "Lighting 101" for a great introduction to lighting.

At the very least, if you're shooting in natural light you'll want to be prepared for different lighting conditions. Visit the site ahead of time and find some spots that will be sunny and some that will be in partial and full shade, so you can control the light. Choose the time of day according to the kind of light you're hoping for. Think about what you'll do if it's a sunny day or an overcast day.