What is the price and how much maintenance history does it have and does the owner seem like a moron?
If the rest of the car is in really good shape it isn't like 4V motors are super expensive at the JY. $1500 gets you a low mileage Aviator motor. Do a compression/leakdown check on it and if it passes then the bottom end is good. If the owner won't do it or doesn't know what that is, tell him you have to assume that there is some issue with the bottom end and make an offer accordingly.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.