1. If we want use only paypal, what we should choose in the "Select your merchant account provider" listbox?\
There is no Paypal entry.\
Paypal account data presents at the bottom.\
2. Is it possible to make and integrate the IPN Paypal module?
Paypal is selected always, if you want to remove it simply remove the code from the templates. Anything you do not want just remove its code from the templates. See the templates for Money manager, payment* etc.