Replacing engine for 1986 528e

I am considering purchasing a 1986 BMW 528e with 300,000 miles on the engine. The owner has records of all repairs through the years, and the car is in good condition. I don't have much automotive knowledge, but would like to find out an estimate of what I should expect if the car needs a new engine. The transmission has about 40k miles, and the car has been very well cared for with all repairs and maintenance performed by the same German car automotive specialist.

BTW, he's asking $1000.

Any ideas or suggestions would be appreciated.