Congrats! You are correct that I built a space out a couple of years ago. I didn't have the high ceilings to work with. Lucky you. As a home builder, my advice is to imagine it it in use. Think about all of the things it will be utilized for, right down to where people will enter, where will the trash go and all of that jazz. You have the advantage of a pre built shell. You and your girlfriend can spend time talking about how it should come together.
Here is what I learned, based on what worked out well and what I wish I'd done differently.
If this will be a play area for you and your partner, you might want to section an area that is only for the two of you and keep it locked during parties. It is something I wish I had done. We sanitize well but we remove our toys and personal items before parties. We have toys for guests but ask they bring their own very personal items, like insertables. Lockers would have been a good thing to have too but I ran myself short on room. Starting out with a thousand square foot for a recreational area sounds like a lot until you start to utilize it.
What we did right:
A full bathroom, of course. We went with a simple shower instead of a tub. All of the towels are white so they can be bleached. There is a paper towel holder mounted to the wall for the simple washing of hands. I don't mean to sound like a germ a phobe. But, there are personal exchanges going on.
We've gotten good use out of a mini kitchen for parties. It is nice to have a full size fridge and everything necessary for set up and clean up, right down to a dishwasher.
Sound proofing if you are close enough to neighbors to need it. We are in a neighborhood where that was important. I splurged on the spray foam insulation. You probably won't need that in the country.
Lighting and a sound system have made a big difference. It lends an element that relaxes everyone. I used a lighting system where the colors can change and dimming or brightening is easy. There are overhead lights that only come on for cleaning.
Parking is something to think about. As with the privacy you will have in the country, it might not be a concern.
You will need contractors for some of these things. Depending on how discreet you need to be, be careful of local ones. You need to consider that you'll have to pull permits. If it is a small town, you probably don't want to draw too much attention. Feel free to reach out to me on private message. I will help however possible.