I'm looking at buying a big block 402 engine that has been setting for 15 years but it has been stored indoors. The current owner has not heard it run but was told that it had previously. He says that it still turns freely. Is an engine like this rebuild-able and how much risk will I be taking?

Assume it needs a complete rebuild, to be safe, the heads should be pulled, and the pan as well, but you can help yourself by inspecting the exhaust ports for evidence of oil and or coolent residue, also look at the intake ports and see if any are wet appearing, depends on what they want, but in the old days I got stung a few times, with cracked cylinder walls, and or bad pistons etc...

Assume it needs a complete rebuild, to be safe, the heads should be pulled, and the pan as well, but you can help yourself by inspecting the exhaust ports for evidence of oil and or coolent residue, also look at the intake ports and see if any are wet appearing, depends on what they want, but in the old days I got stung a few times, with cracked cylinder walls, and or bad pistons etc...

Yeah, cracks would be your biggest problem. The block is basically worthless if it has cracks in it. If the block is solid it should be good for a rebuild. You'll probably need new main bearings, con rod bearings, rings, lifters, and some valve train and head work if you want it to run really good.

Although it might still run strong with some minimal maintenance, just do a careful inspection and maybe ask the guy if you can take off the valve covers to get a better look.