Making an engine

I'm strongly considering making my own engine from directx in c++, and I have a few questions. First, should I make it for Directx 10, with the knowledge that by the time I'm done with it, Directx 11 will already be out. Second: should there be any global functions at all, or should I stick everything in methods within a grand Hierchy of classes that are initialized by loading the engine. Third: how do I go about making an engine DLL-ready, I want it to be all inside a simple DLL that can be quickly plugged into C++, and I happen to have no idea on how DLL's work.

Any help is greatly appreciated.
(PS: I DO know C++...enough...just not DLL's lol)