I'll try and answer what I can, having just replaced my server with the same iMac:
1. Sorry, no experience with toslink over a few feet.
2. Some playback software supports third-party plugins (e.g. Pure Music), while others have built-in parametric eq ability.
3. There are quite a few options, all of which offer trial evaluations periods. Check
here for a list.
4. Airport Express, Apple TV, etc.
5. iPhone, iPad, Screensharing, etc.
Start with your main system and then add on remote systems. It's a somewhat slow, but enjoyable, process. There are many different ways to approach what you're trying to do.