2) You can start with simple stuff. For example Arduino - play with the board, and then start building peripherals around it.
3) For hobby or professional? For hobby I would go with KiCAD, for professional OrCAD or Altium (but you can also try the other software - this really depends if you will be looking for a job or you just need it for yourself).
4) You can have OrCAD Lite for free (it has limitations) or you can start with OrCAD standard for a good price (they keep changing it, find a deal, you can buy it sometimes from 200USD). Altium is expensive 7 to 10000USD, but it is more intuitive and it is used in many small and middle size companies. OrCAD (Allegro) is more complicated to learn and usually used for complex boards (like boards with Intel chips). Many reference boards used to be in OrCAD and Allegro, now you can find also some reference board in Altium.
5) For simple and middle complexity boards you can use almost any PC - I often work on my laptop (Lenovo i7). My office PC is i7, 16GB RAM (but only 9GB of RAM is usually used), no special graphics.
Thank you for subscribing