SubNate,
That used system guamrider is selling may be an excellent way to start. If something has been taken care of there is nothing wrong with going the 2nd hand route. I have been helping people buy and sell their new and used gear for years and I have yet to have a complaint. The equipment simply needs to have been taken care of is all.
That said, with new gear you get warranties and they can be very helpful. Whenever I buy a new cam I always buy a 2nd extended warranty which I always use simply for cleaning and realignment even if nothing goes wrong.
There will be much disagreement with this next statement. Editing has a learning curve, period. No matter what NLE (non linear editing) application you use, there is a learning curve. It takes a long time to learn to use any NLE beyond the very basics. A real editor has spent years learning all the tricks and tips that speed his workflow including the keyboard shortcuts, compositing, color correction, keyframing of effects and so on. When Final Cut Pro first hit the scene with version 1, there were no books, forums, dvds or anything else to learn from and it caused a very steep curve. Mary Lynn Price and I would spend hours on the phone just trying to figure out how to close a gap. Today there is an oversaturation of books, tutorials and dvds of all kinds as well as some excellent forums that you will find will be both helpful and a speeding up of your personal learning curve. Never the less, there is still a curve to learn and if you don't practice, you will forget very quickly. I tell this to all my editing students and too often, after not practicing their editing skills, they call up saying they have forgotten to the point that even their notes don't make sence.
Final Cut Express is a good, inexpensive NLE but is very limited compared to the pro version. Other than the money factor, my question is; Why bother to go through the curve only to outgrow the application and then jump to another, albeit similar, application. If you are serious about learning, start with the NLE that you will probably never outgrow. I've been using FCP since its inception and am Apple certified using it but there remains a ton of things it can do that I do not know how to do. This is because one uses the tools they need and when a new tool is needed, that's when you learn it. While I certainly know the multicam tools, since I rarely ever do a multicam shoot as one would do at a concert, I can't even begin to consider myself an expert using the mulitcam features. Another point is that Final Cut Express does not come with Compressor (needed to encode your final sequences in order to make a DVD) or DVD Studio Pro, where you can make some fairly impressive DVDs, or Motion, a titling and graphic motion application similar to what you get in the more advanced Adobe After Effects or Soundtrack Pro, where you can make your own royalty free music as well as clean up audio problems you may have in your video and finally, Color, which is used for advanced color correction and grading. Final Cut Studio comes with them all.
While Mike is correct that the basic steps for editing SD and HD/HDV/AVCHD are the same....doing the actual editing is very different. It is difficult to quickly scroll through any of these formats except SD...ingestion is different from SD or HDV tape based formats as opposed to HD/AVCHD inter or intra...the codecs and settings are different and even naming of clips has a different workflow. This is not to scare you away from any of this, you WILL LEARN, but step by step, one thing at a time. I'm just saying that if you want to learn, no need to start on a moped before you jump to a motorcycle.
The HDV or AVCHD inexpensive cams would be fine for a starter. As Mike suggested, either the L & M or Gates housings would be more than fine. That would be a choice you would need to make as one(Gates) is mechanical and L & M is electronic. There are also those who swear by their Amphibico housings as well. You certainly want a decent set of underwater video lights, manual focus controls, manual white balance, power, and zoom (you will rarely need to use the zoom control, if you do need it, just get closer) Just avoid the cheap stuff as buyer's remorse never feels good and will cause problems that will only frustrate you.
Good luck to you Nate