How to modify frontend checkout controller

Hi folks,

what is the best way to modify frontend checkout controller of shopwarre through plugin?

 

For example:

  • i want to create second (payment) step as important one, even if user checked its default payment or not
  • i want to create calculation of available payments of the articles in basket and remove the ones that are necessary on the payment methods list
  • i want to make a check if the shippment is available on the customers addres, which depents on articles in basket