GMC SAFARI problem(2003)

My gmc safari worked fine until a few days ago would turn over but not start.I was advised to change thre distributor cap and rotor and I also bought new wires and plugs.The mechanic changed the cap and rotor and the vehicle started first key,he switched it off because I wanted the new wires.After replacing one side the vehicle took a long time to start like when it's flooded but started eventually but with a missing and backfiring sound and yes wires was checked and was installed properly.Now I was told I need a new distributor so I got that installed and now van would start and idle but with no acceleration power to move do you have any idea or suggestion what might be the problem or what might have been the problem in the first place?