Etching is a large part of what I do, I can give you some tips, if you have specific questions feel free to ask.
For one thing, if you're serious about doing hand etching for your rigs or to learn it as an artform at all, forget the dremel. The tool I use is called an Optima II, it's 10,000rpm faster than the top dremel, doesn't overheat, is much more lightweight, the controls are in a seperate box and the etching/engraving/carving tool itself is only a few times larger than a pen and doesn't weigh very much. In all it's a heck of a lot easier to handle and work with and you can get a lot better detail work done with it.
For most of my etching I use sets of diamond lapidary tools that fit the colet of the tool, which is like a dremel's in many ways, you can replace the colet with a larger one if need be as well. The bits go in, you turn the clamp and that's it, you're ready to go. The speed is adjustable from 0 to 45,000 rpm in minute increments, not with a switch, so you can set the speed very precicely.
Now for the down side, the tool runs about $325.00. Which is considerably more than a dremel. But the job it does is vastly superior. It'll do everything a dremel can do and then some though. Imagine a dremel with a flexshaft that never seizes up, never gets hot, is only the thickness of a speaker wire and you don't have to have it hanging over your shoulder most of the time for it to work. Really, it's worth every penny.
Now when it comes to what you intend to etch on, any form of plexi will do, acrylic, lexan, etc and of almost any thickness. But some will yield better results than others. Harder plastics will be able to let you do more fine detail work because often the tool will get hot and melt the softer plastics, causing a lot of pick marks and uneven etching. Hardness of plexi is measured in rockwell, like jewels, metal and other items. The harder the better. These tools are able to etch on diamonds, so you're not likely to find a plastic that's too hard. But you want to be mindful of britelness. If it's too hard AND too thin it might shatter. In most cases you'll not need to even think about this because you won't find a large selection of plastics at Home Depot or Lowes or whatever. You have to go to specialtiy shops for this stuff. Incidentally, if you want to try something really freaky, you can also use this to etch on a case itself, right on the metal, creating some rather interesting patterns. I've tried it and came up with something rather wild looking. No guts in the case though, just an experiment.
As for templating, that's easy enough, you just print out your pattern in reverse, tape it to the side you want to see the image from and etch from the rear. (So lettering should be done in reverse, keep that in mind). The thicker the plexi the more difficult of a time you'll have keeping with the pattern, as the thickness will mean you're etching that far above the actual pattern. I find it's best to work with only 1 good strong light rather than several light sources, as this creates some strange effects when you're etching. And I prefer an overhead light to any light coming in from the sides. Once you try it you'll see what I mean if you use a suitably thick piece of plastic. My own technique is to only use a pattern (if I need to) to do the overall shape, outlines and indication marks and then eyeball it from there, it's actually easier to do it that way and you can stray from the pattern where you need to.
If you're etching a pattern that is not complex but uses very bold lines (say a sillhouette) then you need to be careful about how you're etching. Criss crossing patterns with a finer tip bit will look really bad. I use a ball tip for that kind of thing and minimal pressure. You want to keep the depth of your etch as even as you can. Normally this is a job better suited for laser etchers, but, that's expensive and most of us can't afford it, at least on any kind of scale. It can be done by hand but it's harder than a more complex piece because you want to try to make it look so uniform.
If you plan to use LED's in the plastic, keep in mind that a deeper etch will yeild a better refraction of the light but will also be harder to do complex designs. So you will have to experiment until you find the way that works best for you. I've found that my background in pyrography (woodburning) was very helpful in learning to etch plastic, glass and metal. Both art forms are also very much akin to tattooing. I may get into that some day.
Any questions just ask.