First - you have to completely complete your profile. Completely.
I think almost everyone on this forum has poured over these same aspects of building a machine. I think all of the methods have their respective applications (here we go) depending on the exact design of your motion platform - how much mass are you moving, how fast, how rigid, what are you cutting, etc, etc. ...you know the drill. I am an Engineer - can you tell? Seriously - it DOES depend - I get the feeling you already know this.
I also think that picking a direction is harder than making the chosen direction work - and it can take just as much time.
1) Router/Plasma question - I would suggest that you build for rigidity for routing with considerations for the plasma cutting as you mentioned, then you can do both. There is a guy in Spain that built a really nice plasma table and posted photos recently. The machine resides in his living room....it's the nicest piece of furniture he has! jk..
2) Guiding question - the 3 bearing method is an inexpensive, DIY method that works well. The round bearing thingy is more expensive and maybe a bit better in some respects (I don't like it, though). The linear rails are the way to go if you have deep pockets and want perfect guiding - but I think it depends on what you are looking for and how much of it you want to take on yourself. Many, many people have built pretty accurate, reliable machines with little more than plywood, steppers, UHMW and threaded rod. Others use welded frames, servos, linear rails and ground ballscrews. It depends on your wallet and your ambition.
3) Coupling - when in doubt, I would recommend that you go with the option that offers the most options. The timing belt method is a relatively inexpensive way to get a lot for a little in my opinion. This is my method of choice.
As you know - there are a ton of people on this board with a wealth of info. The most important thing is to take pictures of your progress :banana: and keep us posted!!
Scott
Consistency is a good thing....unless you're consistently an idiot.