I bought my D5100 in July this year and have really enjoyed it - I am very much in the "just learning" phase of this hobby. Snapping lots of pictures and working at getting past the "auto-everything" setting.
I have Nikon's AF-S 55-300 DX VR Zoom Lens which I bought with the camera. I've found it works great with the D5100. I had a Sigma 300 zoom lens from a Nikon SLR but learned it would not autofocus but would fit and 'work' on my D5100 so if auto-focus is a concern you'll need to get a newer lens with that capacity built in.
I did lots of research on line about the flash - thought I'd get away with a cheaper alternative to Nikon's SB-700. But after spending hours mulling it over decided that for my first flash at least I'd get a Nikon. It works flawlessly with my camera and can be the 'master' if I add another flash. The D5100 does not have the flash master control built in like some more expensive Nikon's so the SB-700 (or 900) attached adds that capacity.
I'm using Adobe Elements 10 to catalogue and edit my pictures - again a bit of a learning curve but not too bad. It really can ehance the image in many ways. I recommend it fully.