I would have to say that it entirely depends on the purpose of the acquisition. Not every buy-out is for the same reasons.
For example, you may want to buy out your competitor in order to reduce the competition and plan to EOL most of the products because they are redundant to yours. (Look at what Oracle is doing).
Or, you may be interested in several key patents, copyrights, trademarks etc. and don't actually care about the code (or product) itself.
Is the human resources the key factor behind the deal. Or will the company close shop and move the new business back to head quarters?
Is it for immediate access to existing contracts, or maybe subsidies being received?
It can even be any or all of the above actually. Meaning that the actual documents (architectural/design) and code base 'may' have little to no value to a buyer.
Is it common to examine actual source code? the version control history (the comments or code history)?
Yes it is if the purpose of the acquisition has the objective (in part or in whole) of acquiring the code base. I have been through this process myself once. Some dude showed up at work and interviewed the CTO, architects, project managers and team leads. Then he got access to architectural documents for which he studied for a while and asked to see the code for various parts. This guy didn't care about the programmers at all (competence or expertize). For my modules, he ended up asking my boss instead (non technical), for which he got vague answers. I guess that suited him fine though.
So if your buying, make sure you know exactly what really matters to you. If your being bought, you will kind of get an idea what interests them from where they do the looking.