I don't have schematics for my reader(I'm not that organised), I have the PCB layout and probably can find the component values used. I built my reader based on the information here http://www.compendiumarcana.com/caliper/ and here http://www.shumatech.com/support/chinese_scales.htm. It uses a PIC18F252 clocked at 20MHz which spends most of its time doing not much. My code occupies about 14% of the available code space.
Best way is definitely to buy a scale and start to investigate. I built everything up on a plug board to start with until I was sure the circuit was ok. A digital oscilloscope is really useful too. My reader does nothing clever in terms of DRO functionality. The scales operate in slow mode (~3Hz) and this has been fine for my use. I built a version which switched to fast mode (~50Hz) but found the jitter increased quite a bit. After that I think I got hooked converting my X2 mill to CNC so never progressed the reader as it work ok for me.