Travel / Driving around Poland, is diesel available and can we find lodgings as we go? [22]

My wife and I are leaving for Poland in 2 months and plan to drive ourselves around Poland in a big circular route. Do you recommend gas or diesel automobiles? Is diesel fuel readily available throughout Poland? Will we need to have lodging prearranged throughout the trip or should we be able to find lodging as we proceed from stop to stop. If we avoided Warsaw completely, would we regret it later?