When do I need to replace engine or transmission?

That car with those many miles is worth $500. There's no telling how it was maintained and why it's up for sale now. Something may have been noticed and is a sign of the rains to come and has become the reason to sell it. DO NOT believe that the car is in great shape just because it looks good. You'll find out the problems after you own it because the seller will not admit to them now. Unless you have the time and some knowledge to fix auto problems then don't bother with it.
Too many people think they have gold and don't realize it's a lump of coal. It seems to me that the seller's asking $2000 too much because they want to be paid for the oil and filter change, last car wash, gas fill-up, air freshener, ..., Etc.