Some reading material for you. You can buy almost everything off aliexpress and ebay just search. You can buy a rtk board at ardusimple.
For a basic system you need:
-Base station (you can build one or find the closest one to you, highly recommend build your own)
-RTK gps board (Ardusimple simpleRTK board)
-Tablet (windows 10, recommend bright screen)
-Motor controller (There are two: IBT2 or Cytron MD13)
-Motor (24V motor)
-PCB for autosteer (For beginner use official V2 board or Kapoui, you can find the files in the support folder) with the PCB you also need resistors, capacitors, diodes, arduino nano, ADS 16bit
-Wheel angle sensor (Delphi ER10031 or rqh100030 are the most common)
-BNO0800, CMPS14 or another RTK board for heading and roll
-Step up converters, i use one for stepping up the voltage for the motor and depending on what tablet you buy, you can use a step up or step down converter to charge the tablet
Some links for shopping (These are all free shipping to my country, fell free to search for youself)