Yeah no worries!
But still take another look into the car, if it runs well and a tad quicker then standard, in a good condition...
Do a bit more research, ask a few more questions and even take you mechanic down with you next time to see the car.
The seller is negotiable on price so take a stab at it... He wants $18,499, then see if he will take anything in the $16K range.
If not then you both meet each other half way on the price, say $17,500 or whatever...
If he won't lower the price, meaning he still thinks his car has some good mods and can make 239kW's. (Trying to get some money back on the "Mods" he has done")
If he is willing to lower it then you know it's pretty much stock...
My point of view anyway and this is all in speculation, cuz I am not standing in front of the car and just judging from the Carsales add.