Hate to disagree with you, but there are many other things that would be far better in my opinion. I learned to drive a car by getting behind the wheel, I learned to drive a Skid Steer and a Excavator the same way, professional Training? nope, never had it and I was a lot better than some of the guys that had it.
Never used a hand plough but lets be real, how much training would you really need to push a blade in a strait line? as for the tractor, I have used real tractors with no professional training, lets face it, it's an oversized lawn mower with a PTO to the back, put it in gear, drop the attachment, engage the POT and drive. Takes practice to do a great job, but anyone can do it without any training.
That said, I do think that if you are not a Farmer then you should not be able to just up and use farming equipment without a penalty of some sort and the same with the Excavator, the skid steer, well, any joe blow can rent and use one just about anywhere in the US, granted they are no where near as strong as they are in the game but still.
I think linking the equipment to the associated Skill should be sufficient, but even that Idea I don't like as I am not a professional in any field that uses the above equipment, but yet I can, and have jumped on them and used them, I also 100% disagree with requiring a skill to be unlocked for the trucks, my 13 year old can get in the car and drive and she has never had any training on it.