The place that you really are looking for is a mountain that faces North or Northeast. The 3000ft. or 4000ft. altitude is great. There are some things that can cause problems even at that altitude. Look for the right companion plants and trees. Poplars and Maple work real well. If there is Black Cohosh and ferns that makes it even better. If you see lots of scrub type Oaks and Pines, that can be a problem even at 3000ft. If the slope is facing West, that can be a problem. If you're investing good sums of money for land, you'd better have all the ducks in a row for the best results. Walk over the property good and see just what is growing. The altitude won't make too much difference about diseases. Moisture levels will. Good luck.
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.