although typically the robot enthusiast groups deal with wimpy little robots. But, it's likely some have good control experience, analog experience, and motor control experience.
Try looking up PID control (Proportional Integral Derivative) - that's what is normally used for closed loop control, e.g. on all the servo systems we build (and for temperature control, etc) (there's also fuzzy logic). We use commercial motion controllers or PLC's to do the real time stuff.
You'll have to interface to the feedback system (analog front end plus A/D converter to get from resistance to voltage to bits) and jack screw (motor driver of some sort).
I'd recommend using something with a bit of computing horsepower so it's easier to write the NMEA code. There are plenty of little boards available; if you want to use something other than assembly, BASIC, or C/C++, get something that gcc runs on (e.g. x86, PowerPC, ARM) and you should be able to use Objective C via gcc.
Also, it's likely you can visit eBay or other surplus sites to get commercial grade stuff (e.g. brush or brushless motor amplifier) to make your life easier but still be affordable.
--Tony