1) OpenGL is pretty advanced, and not really something a beginner programmer can just "hop into." To write your own engine you would need to understand vector/matrix math, GLSL, NIO buffers, texturing, GL blend functions, etc.
Why don't you develop your engine on top of Slick2D or LibGDX?
Alternatively, if you're keen to dabble in OpenGL, you could use Slick2D only as a base for window/game loop/etc, and write your own OpenGL texture loader, sprite renderer, etc.
2) Depends on what your definition of 2.5D is. Regardless, this too sounds pretty ambitious for a beginner.
My advice: dive in and start programming, and you will soon realize whether or not this project is feasible for your skill level.