Am I reading you right, 4 years-15k? 4 years-15,000 mikes? If so, any internal gearhub should have no problem with that. Sram, Old sturmey archer, shimano any of them will work fine. Choose how many speeds you want and then select your model. Your concerns about the nexus 8 being unproven should be calmed a bit by realizing that it is merely an updated version of a nexus 7 which has been in sevice for many years serving cyclists worldwide. All internal gear hubs work off the same concept. A main gear called a sun gear surrounded by smaller gears called planetary gears. When the shifter cable moves it causes the sun gear to slide over to the next set of planetary gears thus changing the ratio or gear. Most all heavy industrial equipment in the world uses a form of planetary gears in operation. Automatic transmissions in autos use a form of sun gear. All old technology and well proven. If you are not aware of it there are two versions of the nexus 8. the standard and the premium. They share the same ratios but the premium has ultagra quaility bearings and seals. It is also lighter in weight and would be my choice. Check out Sheldon somewhat eccentric, fix most anything, been around a while, gearhub Brown's website. Browse around read his many articles on all kinds of gearhubs,and mechanical tips, Official shimano disassembly of Nexus 8(vidio). Don't overlook his article on how to make your chain and sprockets last longer. Here is a link to his latest 8speed bike for starters.
http://sheldonbrown.com/harris/bianc...os8/index.html Here is a direct link to his internal gearhub article.
http://sheldonbrown.com/internal-gears.html