Hello, If I buy a water heater from there will you guys install it, and how much will it cost?

Better to ask

Home Depot uses independent contractors to do installation's if you need one. Get a quote from them and compare it to one of your own choice. Check reviews for both and that will help you make an informed decision.

If they don't, I'm sure if you ask them, when you purchase it they can hook you up with someone who can install it.