My first Technical Lego kit I ever got my hands on as an 8 year old was the 851 tractor. I can still recall looking at it in the Toy shop window and really wanting it for Christmas. I've still got the kit, box, instructions and all the parts.
It features real working rack-and-pinion steering through a universal joint and power transmission to tools. The tools can be attached to the rear of the tractor and raised/lowered via a lever on the right of side of steering wheel. A true classic Technical Lego model.
Scans of the plans for the 851 can be found at Peeron.com
Here's some Lego leaflet blurb dating back 33 years ago about the 851 tractor:
For old time's sake, I've assembled it and stop-frame animated the assembly in the video below: