Welcome! It is always nice to meet a new person who shares the same interest as myself. I also started with a Stewmac kit and am very happy I did. The instructions are fantastic and between the video and manual pretty much every question I had was answered clearly. I am building an LMI kit now and although the kit itself is top notch, the instructions are somewhat lacking compared to the Stewmac kit. I refer to the Stewmac instructions and video often while building the LMI kit. That is not to say the LMI kit is bad, it is just that the instructions assume that you already know something about guitar building. They skip a lot of details that Stewmac covers. I have to say that LMI has them all beat hands down when it comes to kit selection. With their kit wizard, you can build pretty much anything you can think of with any kind of material you want. It is amazing. As far as Martin, I have never built one but from what I have read, the kit itself is good but the instructions are poor at best. You will definately need a lot more instruction material than what comes with the kit. Read Bill's comparison of the 'big 3' kit supplier manuals. You will find that he agrees that as far as instructions go, the order is Stewmac, LMI, and in a very distant 3'rd Martin. Bill's site is what turned me into the direction of Stewmac as a first kit. His site is a very valuable reference tool that I frequently visit. Of course the forums are a wealth of information too.
Anyways, have fun, ask lots of questions, and post pictures of your progress. Don't be a stranger!