300-400 acres is plenty for an 815. You say it has a newer motor, what is it? The 15' head is a good match for the combine. A 4 row corn head works nice with the 815 also. We had an 815 several years ago and upgraded to a 915. The 915 was a much better combine. It had more capacity and seemed to do a much better job. Our 815 had a gas engine which sucked, hopefully the one you are looking at has a diesel engine.