My team has been hired for this role several times: we receive full access to the technology stack and the target's engineering team. We then prepare a thorough report that the potential acquirer can ask us questions about, discuss any concerns, and request potential solutions to any issues and associated costs.
This has at least three benefits:
1. Isolating the target from revealing their IP to the acquirer,
2. We're experts in the target's stack (say Rails), whereas the potential acquirer may be more of a Java or Django shop, and be unaware of stack-specific issues to look for,
3. Allowing us to offer an unbiased opinion: we don't have any vested or emotional interest in the sale going through or not, so we can be pretty blunt about anything that might be an ongoing issue.
I won't say how much we charge for a service like this, but it's absolute flea spit compared to the $14 million on the table.