Practice with the lens that's on it. You might be surprised by the results of this 'normal' focal length in large format.
My best advice to you is to use 'string focusing'. Basically, attach a piece of string to your camera, and put markers on the string that represents certain distances, and then focus on the markers to figure out what distance that is. When you pose your model, use the string to quickly focus the camera, and you can have the film holder already loaded this way. It's very accurate, and very fast. And it's a lot easier to capture 'fleeting moments' this way, as opposed to using the ground glass where many moments come and go between focusing and actually exposing.
If and when you decide to replace the lens, basically the whole lens board in front comes off with both shutter and lens attached to it. Graflex Crown Graphic cameras had shutters specifically made for the lenses that came with them, and are an odd size, so you may have to have adapters made to fit exactly your lens of choice on it.
Attached is a portrait made with a 5" (127mm) Ektar, at f/5.6 and a Crown Graphic. That's wider than the lens you have.
Have fun, and good luck!