Generally, you pay the rate they state per hour - it will be on tach or Hobbs - it makes a difference - some will specify a daily minimum - one would not want to rent a plane for a week and find it only had two hours put on it.
You should buy renter insurance no matter what the renter may say about his cover - basically think of it as "you bent it, you bought it".
Also really you should try to rent from someone with whom you expect to have an ongoing relationship - not a stranger.
I understand that the Blackpool guy who was jailed after the accident with the overloaded PA28 used to do a lot of renting to strangers
