Since I'm affiliated with Zen Toolworks, this reply may be taken as a bias point-of-view, but I swear to be truthful...
If it helps, my 4-year old daughter assembled the 1212 and 0707 with me. She seemed to get it, unlike my 6-year old boy who couldn't keep his eyes off of the TV.
Documentation is available online and so is an immense amount of support from both ZTW and the community (forums.zentoolworks.com). It should be pretty dummy-proof.
The spindle is awesome. The CNC and spindles are purposed for soft elements and general wood carving. We don't really support metal work and usually suggest using a Milling machine instead.
Drivers are where people start to get confused or misled. 9.999 times out of 10, any problem with either A) the driver card, -or- B) the axis motors, will be due to configuration and not hardware. I would research heavily on which driver to use. Obviously you will need a 3axis card but think ahead -- will you be adding another sooner or later? All drivers run hot I think. Its the motors you want to keep from being too hot which is controlled through correct or incorrect driver configuration. Anyway, we test each 3axis card prior to shipping to ensure they work as expected.
Stainless steel guide rods and fasteners do not rust. The lead screw or the fastening screws are the only thing that may be subject to rusting and they respond the same as any other screw of same character. The wood screws have a coating on them from the factory. I would judge this based upon the how metals respond in your area.
I have yet to see any CNC system big or small compare to the prices at ZTW. Most businesses are focused on growing the business, but we're focused on growing the community (at almost any cost).
Oh, I too would love to have the height extended. In fact, I'd love to have it where the customer can simply enter how much travel they want on each axis. We do have an extension for the X-axis (7" to 12"). These options will come sooner or later, but no official statements can be made about them yet.
However, our foremost concern is the system balance and load path. These are very important since the basic structure is strong or weak based upon where the loadpath is on each axis. If we change the height, even a little bit, the loadpath changes which will most likely weaken another area of the frame or axis. Another issue is time. We all have hectic full time jobs because the prices are low enough to basically cover operating costs, and that's it.
Bottom line, this is a DIY kit and you are free to hack it up to your liking. It's purposed for beginners and professionals who need something that simply works that they can customize at will. So, extend each axis as you want for testing. If it doesn't work, you can always set it back to the way it was. Use any frame-based material that is study and non-flexible and post questions and progress on the active forums where you will receive feedback and advice. Who knows, maybe you'll come up with the official "home-made" Z-axis extension for the Zen DIY CNC Kits.