I dint suggest to use the checkout wizard.
I suggested creating the checkout form only.
you can remove any of the form elements that code is creating that you dont want to use.
"What is the difference between <%spCART_Index = spCART.DisplayIndex - 2%> and <%spCART_Index = spCART_Index + 1%>?"
none. it's a different way to do the same thing.
the 4 new variables that are created are used for passing variable data to paypal, if you dont need them, then you can remove them.