2000 mustang misfire cylinder 5 help

Hey just bought a 2000 v6 from a dealer 150K. when i bought it checked the fluids n stuff all ok oil about a quart over filled so did oils change first thing. Also when i did test drive noticed it was jumpin alot like it was missin but no check engin light was on. So i bought anyways and as sson as i pulled off the lot light comes on so i changed all the spark plugs and 2 wires need replaced as well. Noticed middle plug (5) smelled strongly on gas, none of the others did it was also one of the ones needing a new wire. So i reset the codes and ran the car it blinked out the code and then stayed off? Got late so i just came home. I asked around some one said oring in that injector could need replaced? really not sure what to think any advice on something else to check or do, do i need to change the fuel injector? is this going to be a long/hard process also what do shops usually charge i have never put a car in a shop please help in kc area