For discs and internal gear hub you really want either sliding dropouts (Civia) or eccentric bottom bracket combined with vertical drops, so that brake calipers and rotors are aligned.
If going for the Sturmey 8 (XRK8), make sure you get the new model (coming after July) with its better shift mechanicsm, wider gear range, and promised better durability.
My personal suggestion if you want internal gear hub and discs is Sram I-Motion 9. More durable than Sturmey 8, wider gear range, quick release cable connection, and you can run normal size chainrings because direct drive is 5th gear instead of 1st.
Now, if some of the frames that interest you don't support discs, but do have horizontal dropouts, consider a rear drum brake. Based on my experience, I highly suggest the Sram S7 Drum brake unit. You can run a disc in the front, where it matters, and have excellent sealed braking in the rear.
BTW, the very durable Sram S7 is running about $150+$25 for the shifter right now, compared to $120 for the Sturmey 8 disc and $230+$35 for the shifter for the Sram I-Motion 9 disc (
www.aebike.com)
Should you also be interested in the Shimano Alfine, you can get them for $230 +$35 for the shifter from
www.hiawathacyclery.com