How much will it cost to replace the engine?

What is your question?

I am thinking about buy a vue from someone. It's a 2002 but there is a knock in the engine. I'm looking to spend around 3500 on a new car total. He is selling it for 1800. Found an engine for 500 with a 3 year warrenty. Just wondering if it will still be in my price range to have it put in. Really like the car and if I can stay under 4000 then I will definitely put the work into it and buy it.