Before we respond, I do want to give attention to Basement_Gainz and his "dissenting opinion" above. I am no legal expert but that's probably worth broaching with your dad to make sure he's considered the ramifications and still prefers the idea of creating a gym at your facility over the simpler way: offering paid-for memberships at a nearby commercial gym. Philosophically, I love the idea of an office gym and know a couple examples where it works, but those were very large companies with a few gazillion dollars for whom the legal liabilities were not a major issue, and I do not know anything about the degree of extra insurance premiums your father will have to carry or if there's a way to ensure that all employees using the gym release your father of ANY liability by signing a waiver. Might wanna chat with a lawyer about this.
Anyways...if you forge ahead with the "gym at the office"
Agree with this. Even if some people are overzealous in estimating how often they'll use things, it will give you a general sense of how many people care and what they most want to see in a gym (note: response rates to surveys are generally terrible, but in this case that will be useful - the employees that take the time to fill out and respond to the survey will be the ones who care most about using the gym, so the feedback you do get will likely come from the people who will use it).
This would also show your dad that you're taking the planning seriously, and may be useful in justifying the specific items you ultimately purchase. You need not worry as appearing to be a kid with a rich daddy who was getting a $10K lesson in business; it was obvious to the upright-walking mammals posting in this thread that you meant your father, a business owner, would offer roughly a $10K budget to equip an office gym because he felt that this was potentially a worthwhile perk to offer his employees, and he had given you the responsibility of outlining the equipment which could fit within said budget.
Oh, but for one thing in Emily's post: it's a "kettlebell" not "kettle ball" - and if you say that again, Emily, so help me God, I will bring down the fire on you, too. As carbidiass just reminded us all, THIS IS NOT A SAFE SPACE. It's TESTOSTERONE NATION, baby. Do NOT come into this house if you can't take the heat. IT'S THE ONLY WAY WE GET STRONGER. Flogging each other on the Internet shapes better people. Learning is a painful process. Without paint there can be no learning, a great philosopher once said. All this shit makes for a better person, you feel me? I hope you are a better person as a result of this Internet-beatdown over your use of the wrong term for kettlebell. Go forth and harden up.