How do i know they are telling me the truth? peugeot 206?

I got an engine diagnostic light on my car
My car has been sat still for 3 weeks, started it on friday, tried to pull out of junction, i had very little power light came on think its in limp mode, took to garage, now they are telling me i need a new air filter and coil, because its been flooded. How can the air filter become flooded wsat still? How can this affect the coil?

How do I know they are telling me the truth and not just trying to charge me for bits i don't need>