This statement \"I know that seng is growing on it\".. is absolutely the best assurance you can get.
I own 30 acres here at my home place and have tested plantings in several hollows, some that had wild seng growing already, and some that did not.
My best results are showing up as expected... in the hollow that already had some wild seng.
Many other locations that looked promising... but had no wild seng growing... have not done well at all.
If I were you I would look the property over really well and see how much wild seng is there, notice the size and health of the plants, and notice if it looks like it is reproducing well (older plants with offspring). If possible, I would harvest a few mature looking plants and see how old it is, and check the root size. That may or may not be legal depending on what state you are in. In TN it is legal to harvest for your own personal use (outside the official harvest season), but not for resale.
Look for signs of disease... here on my place the first indication of fungal issues is little yellow spots on the leaves... which later turn into bullseye type spots, and then to leaves that look like they are melting (later on in advanced stage).
I think the elevation that seng does well at varies by location some. Here where I live (southern middle TN) our elevation ranges from 600' to 1100' (in my county) and we hardly ever find seng above 800' elevation. We don't have mountains here, just hills and hollows.
Mountains obviously change that...
Best of luck with the location and your decision.