well, usually, when we buy a second hand car, we change all its fluids first like engine oil, coolant, etc because we can never be sure what grade or when the previous owner has changed that .. but if you are sure then that's fine..
secondly, i'm not saying engine is "farigh", but it would definitely need to be checked how much damage has been done IF there is some...
can start from as low as a couple of thousands and can go up to 60-90 thousands in repairing cost of an engine. depending upon what parts are needed to be changed and how much labor it requires.
khair..
get the oil change first ..
and as other member also suggested, get the compression test done..
get second and third opinions from reputable workshops : /