Do not worry about the oil leaking out of the hub. Enough oil remains in the hub to keep it well lubricated until your next scheduled oil change. You can wipe off any excess oil and dirt that accumulates. I use disposable paper towels on occasion for that.
Do not worry about water leaking into the hub from rain, as long as you carry out the scheduled oil changes and do not immerse your hub in water when fording a stream or creek. I have taken my Rohloff bike on a couple of tours that were a month long in very wet weather. When I changed my oil later, there was no sign that any water had gotten into the hub.
Make sure that the wheel builder knows how to build up a Rohloff wheel, confirm that they read the Rohloff instructions. For example, a standard wheel is laced for a three cross spoke pattern, but a Rohloff for your size wheel would be two cross.
Thorn uses a 17 tooth sprocket (odd number of teeth) on the hub unless the buyer specifies otherwise. Rohloff sells hubs with 16 tooth (even number). If you get a hub with an even number of teeth, read this.
https://www.sheldonbrown.com/chain-life.htmlI cut a small notch in one tooth on my sprocket and one tooth on my chainring so that every time I put the chain back on the wheel, a link with outer plates goes on those notched teeth, as noted in that article.
You will need to decide what size chain ring you want, change the teeth on that and the entire gear range goes up or down. Around home I use a 44T chainring, but when on a tour with a heavily loaded bike I use a 36T chainring. This is on a bike with 26 inch wheels and 57mm wide tires. I have to add or subtract four chain links to make the change.
An FYI, you are used to having taught shifter cables. But the indexing is in the hub on a Rohloff, one cable is used to downshift and one to upshift, you need to keep those cables slack. If you have your cables too tight you will have shifting problems. I have about a half gear change of slack in each cable.
If you want a disc brake, budget separately for a Rohloff pattern disc.
I do not know if you use drop bars, if you do, there are different ways to mount your shifter. I tried several locations, eventually decided on a hubbub adapter and put my shifter on the right side bar end.