A year or three ago I would have agreed with you about video. Now? Sorry - buying a dedicated camcorder is a waste of money.
It really depends on your budget. The recently announced Sony A7r2 will be $3200 and will be crazy good. The current A7-2 is about $1700 and does everything you need except the silent shutter (it is quieter than most dSLRs, but not silent - the A7r2 is). If you really want crazy low light shooting, then the A7s will get you that as well as silent shutter. Downside is less file resolution (12 MP) but if your'e not printing large that is a total non-issue except to pixel peepers. With any of the Sonys you can adapt your Nikon lenses or get some of the native FE mount stuff. The 55/1.8 is a stellar lens and the new Batis stuff is ridiculous (24mm and 85mm).
I shot the Oly for a year or so. The sensor is much smaller than full frame - u4/3. APS-C is in-between u4/3 and FF. Oly makes some great glass though - the 75mm/1.8 is flat out amazing, and is a 150mm equivalent as the u4/3 has a 2x crop factor.
If you want to get more background info, I think these forums are the best. Not so many wankers or trolls -
getDPI | Photography at its best
You will need to learn Lightroom or Photoshop. These apps are the equivalent of today's darkroom. Just like with film, part of the process was getting the shot, the other part was developing. These apps are how you "develop."