Welcome to the club, purchasing a car like this is probably a better idea for someone that is more experienced in mechanics (no offence)
but fear not! This forum is full of people with a great knowledge base.

Last I checked the spark plugs are good for 100K miles. Transmission fluid could be gunked up need to be replaced. We need more info on your vehicle.
Auto or Manual?
As far as the vibration during idle I had the same issue in my '05 limited 6 speed which also caused the rpm's to spike between shifting and whenever the car went into neutral the rpm's would spike for a second then go to normal idle.

I inspected under the hood with the intake cover still on and I could hear some hissing. turned out the hose from the throttle body to one of the valve covers was worn out and had cracks in it. I went to Autozone and spent $2 for a foot of hose and haven't had the issue again.

I would send pictures if you still had trouble but my car is on a boat to come join me here in Guam.

Good Luck!