I have a commercially manufactured optical trace machine- I bought it new in about 1992 from C&G systems, which is a division of thermal dynamics. They mainly make CNC units, but they still make the optical units as well.
A new optical trace unit, with table, runs around 10 grand. I am not sure how you make one of something for $500 that a factory making lots of em sells for 20 times that- I guess if you get everything for free, and your time is worth nothing.
Anyway, as metalmuncher says, virtually every optical trace unit ever made uses the westinghouse design electric eye. You might find one of these used- but I am sure, new, they are a grand or so.
Then you need an electronic controller. Not a computer, but a custom made controller that converts the signals from the electric eye into voltages that control the motors that run the torch in the x and y axis. On my machine, this is a circuit board about a foot square- a few IC's, and a bunch of other miscellany. Then you need the mechanicals- racks and gears, frames and drive motors. Mine also has an electronic torch height feedback system, and a third axis motor- I consider this essential for serious cutting, as the metal warps from the heat of cutting, and will throw the torch off registration if you dont have it.
Add it all up, and its pretty complicated, and, since they are essentially obsolete in industry, a lot of the parts are not readily available.
I have little problems with patterns- I can draw a pattern by hand much quicker than I can debug an autocad file, and I use 4 foot wide butcher paper, at something like $50 per 500ft roll. I buy Sharpies by the dozen, and when I have a mistake, or even some dirt on the pattern, I find white-out works great.
But to build one from scratch would be a lot of work.
I would recommend either trying to find one used, cheap, or building a CNC version instead- the parts are actually cheaper, and easier to find in this digital age. You can go to
www.CNCZone.com and find a bunch of guys who have done it- my guess is parts are closer to a grand, and build times vary. There are companies that sell kits as well.