I can't tell you anything about using external motion controllers with LinuxCNC as I use it via a parallel port (in fact, an add-on PCI card in my case as my motherboard has no native parallel port, like many modern motherboards). However, the motherboard is a fairly basic AMD-based board which works well, within the limits demanded by my fairly ratty CNC router. Installing L-CNC is, in practice, no more difficult than installing Mach3, in my opinion. Find the web site, follow the instructions, and you will have a working L-CNC system. Basically, you download the system image (using any old PC), write it to a memory stick, take that to your target machine, and boot from it. When it's finished, you'll have a working L-CNC system. As you say, there are wizards for configuring the software for a particular machine and from reading this forum, I would say that there are more problems with understanding how your machine is wired and connected, and things like "steps per unit", than there are with understanding the software and configuring it. From this perspective, I don't think that there is much difference between Mach3 and L-CNC.

Sorry I can't help you with comments about external motion control boards and L-CNC; when I first started using it around 3-4 years ago, I don't think that these existed apart from the MESA cards, and they looked complicated and expensive. I haven't really bothered to look since.

So, I don't believe that L-CNC does need particular in-depth computing skills as long as you aren't using the same machine for other software. And you shouldn't do that (or, at least, not at the same time as you are CNC machining! Although I do admit that I might have, say, an MP3 player application running at the same time as I'm using the router and never seen problems with it. But don't tell anyone that I said so or half the membership of this forum will publicly flay me alive...)