Hi Tom,
Nothing wrong with a 12" scope as your first. My first DIY was a 10", and I've just finished a 17.5". Another IIS member, Hickny, is working on his first too, a rebuild of a 13.1".
There is one book that you should read before you start though:
"the Dobsonian Telescope, a practical manual for building large apeture telescopes" by Berry and Kriege.
It will explain how and why a dob works, along with the materials, formulae, ideas, etc. You can get it new form Bintel and a couple of other retailers, or you might want to post a 'wanted' ad for someone's.
Though a little dated in the design aspect, every single dob that works, no matter what it looks like, obeys the same principle spelt out in this incredible book. Where it is thier 8 pole truss scope, or down to a single pole beauty, they all work the way they should only if the mechanics are obeyed- you can't run away from physics.
Here in Oz, very, very few people make their own optics. Most either purchase the optics and maybe a few of the other bits and pieces. Other than the optics, it's upto you just how much of the scope you'd like to make. Others purchase a secondhand scope and rebuild it, like Hickny, and myself with the 17.5".
Hickny and I, once his scope is done, will look into building a 12" together. I've picked up a 12" solid tube scope, and we'll get onto its redesign soon. The scope building bug has bitten really, really hard!
Here is a link to a very good ATM resource for components and inspiration. Have a squiz:
http://stellafane.org/misc/links.html
Half the fun in making the scope is coming up with its design and planning out. This is the only thing I'll suggest you do- take your time with the design and planning. It will leave less to chance and reduces errors. The planning and design stage of my big scope to close to a year. Building happened over 4 months, between family and work, with one month nothing being done.