If you're computer savvy and electronics savvy you can do it yourself with components from China for not a ton of money. Search threads below for ideas. 

If you want to buy something from an Ag brand that has support, Deere, Ag Leader, and Trimble (but some areas have no support from them) are the most likely candidates. 

What else do you have for equipment? Would you eventually move stuff to a combine or sprayer or anything else? Planter control? Or just want to steer the tractor and that's it. 

Most in your spot would put a steering column mounted electric motor for steering. (main piece #1) Then you'd need a display, usually touch screen to do all the calibrations, making guidance lines, etc. (main piece #2) And last a GPS receiver on the roof. (main piece #3) 

John Deere ATU 300 is the current steering motor, there are older or used models the 200 and 100. Ag Leader Steady Steer or older would be OnTrac 3 and lower number. Display examples would be John Deere 2600, 2630, or higher number gets newer. Ag Leader display models would be Integra, Versa, for used or there are newer ones. GPS receiver would be Starfire 3000 or 6000 for Deere used, higher number for newer. Ag Leader could be 6000, 6500, and higher numbers. 

As you can see. Lots of options. Who is a local dealer you like or trust for support? Nice thing about the JD or AL stuff listed is you can move it to other machines. Hook to planters for clutch control and population monitoring. Sprayers for section control and rate control. Combines for yield monitoring and steering.