Ask HN: Big company approached our stealth startup – what to do?

We are a 3-person startup a few months away from a public launch. We got under the radar of a very big company in the same industry we are operating in. They have been looking at offering a service/product similar to ours and are now wanting to talk to us. We have an NDA in place and have already started preliminary discussions which are now intensifying. I’m not sure what their end game is. It’s not that they are being coy about it, it’s just that they first want to see if there’s room for doing something together in some capacity.

They seem to be interested in launching it under their own brand, so I think a simple investment is out of the picture. I think it’s going to go down as an acquihire of some sort. We are moving into the more detailed stage of a technical due diligence type discussion, and we're not sure to what extent we should allow that exploratory effort go. We don't really mind talking about our infrastructure and business aspects, but is it considered acceptable to let them explore the code in great detail? Again, it's a very big and reputable company, so I don't know if we should be so paranoid as to make them walk away.

Should we have in place a more specific NDA or a letter of intent of some kind? We are open to doing a joint venture or even an outright acquisition as they have the right resources to take our product very big very fast.

Any tips or suggestions?