In my personal opinion, the best way to keep the pests at bay is to keep the coop clean and ventilated. If the birds are happy and healthy, they'll be less susceptible to the nasties. Make sure they have clean litter, clean water (ideally with a bit of apple cider vinegar, so I've been told) and LOTS and LOTS of ventilation.
Be prepared to lock the new girls in for the first few days to week so that they get used to the coop and learn that it's "home" - this way they'll go back to the coop on their own at night, so you won't have to chase them down! I feed mine several times a day during this period, so they learn to associate me with food. Now my older girls come running as soon as they see me!
Treats are also great during this initial period - oatmeal, grapes, lettuce, cooked pasta, yogurt, etc. Mine get most of our leftovers.
Depending on your management plan (pen, free range, etc) you may want to clip their wings. My girls have been known to try to go visit the neighbors (over a 6-foot fence), and before I clipped their wings they were absolutely capable. Grab them by the feet, turn them upside-down and wait - they'll calm down in a few seconds to a minute, and spread their wings out for you. Just clip the outermost row of feathers. They'll still be able to hop (mine can still get up onto a roost 3 feet off the ground) but they won't be able to get over the fence.
I recommend a fishing net or poultry hook to catch any escapees (it happens to the best of us every once in a while!)
Everything I've heard on oyster shell and grit is that they are "as needed" equipment. If your hens free range, they will pick up pebbles and sand from the dirt as necessary. If they are in a pen without a dirt floor, grit should be provided. Oyster shell is supplemental calcium used to toughen up eggshells. I have never had a thin-shelled egg, so I don't offer it to my hens. I do, however, keep eggshells so that I can provide these to the girls as a calcium substitute if it ever becomes necessary.
I haven't been doing this too long, but that's the wisdom I've collected from all of the old-timers that I've talked to.
Good luck, and HAVE FUN!! Don't let yourself get too stressed over it all - they're hardy critters and they'll do just fine.