If you would like to buy some components or a ready-made set, there is a Polish store https://navisklep.pl/
the basic things are a tablet, box/controller, turning sensor and GPS antenna, if you assume that the whole thing will be with an auto steering system, you have to answer yourself what you want to turn the steering wheel - a DC motor with a plastic gear mounted on the steering wheel / electric steering wheel / or a hydraulic block - there are different opinions on these things, for example a DC motor with a gear may be the cheapest, assuming you have your own 3D printer but without a printer, I think it will also be a cheaper option
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How to get started
I don't know where to start
Starting with agopengps