If you're past soldering DS80C400, you're home. Software would be less than 100 lines of ASM (plus 64K of TCP/IP stack but it's already there, in ROM.) Some extra hardware, like the motor drivers and camera signal conversion ADC, a bit of generic software for PC, and it's done. Of course if you wanted to build it on a generic '51 clone, you'd have to write the stack by yourself (but if the DS guys did it, why couldn't you?), attach some ethernet drivers etc. Difficult. But if you manage to attach the 100LQFP chip in a sensible way, the rest is a piece of cake.
BTW, don't they sell DS80C400 development kits?

BTW, have a look at Atmel website, microcontrollers/networking. Looks like a vaporware but such a kit should be easy to modify for such a project...