You are taking a gamble with any used rental equipment like you would on a vehicle as far as dual fuel they tend to give more issues than a dedicated fuel system
You can run just Lp and not use the gas
As far as Lp versus gas ,Lp is more reliable, burns cleaner and does not cause build up on plugs etc. as the run time on tank of lp it varies depending on your work load size of tank etc but generally
Around 6-8 hrs on a tank providing you are using a standard size tank