Hi Jeff,
Looks like a cool project!
I'm not sure of your experience with the following so bear with me if nothing is new, it may help others though
As it is a very interactive application you'll definitely be looking at javascript on the client side with data feeds and updates to/from the server via ajax to give the user a seamless experience and avoid page refreshes (jQuery).
For the graphics you can look into html5 canvas or WebGL with the html5 canvas probably being the easiest for a 2d schematic type drawing.
For the server side, it doesn't matter what tech you use as long as you are familiar with it. If you want to use javascript on both ends you can use node.js on the server side for instance.
So for your client side tech:
html5 with canvas or WebGL (THREE.js is a good library over WebGL)
jQuery (for dom manipulation and event handling and some UI goodies)
javascript (I use the 'modular' pattern approach and avoid frameworks personally)
server side:
whatever stack you are familiar with, perhaps with a RESTful api (but not essential and can add un-needed complexity).
MySQL or XML/JSON for image and data storage or a mix of both (XML/JSON for image geometry, db for object property data)
The hardest part is building the drawing engine as you will need to start from the very basics of capturing a drawing context, adding primitives, handling user input and serialising the geometry and data.
All the things we have in CAD built in already but there would no doubt be third party controls ready to go too.