Negotiating Acquisition of Copyright for a Book

There is a book in a specialized field that I think would sell reasonably well if it were updated, redesigned, and republished.
The book was originally published in the U.S. in 1979, and afaik the original publication company still owns the rights to the book.
I am considering getting in touch with them regarding my desire to acquire the rights to the book. I believe that there are a few publishing companies that will have an interest in it, but I would prefer to acquire the rights myself (as an individual) and then sell the rights to the book only after revising and redesigning it.
My issue, however, is that I have no experience with any of this.
What I want to do is approach the original publisher and ask them if they are interested in selling the rights to the book. But, I don't know how to approach this in a way that I don't overpay.
Let's say that the book was expected to sell 5,000 copies @ $20 per copy. How much should I be expecting to pay for the copyright? I myself don't have a publishing company, so I will still only be getting a cut of the profits after all the work is complete and the book is published.
Also, is it even possible for me to buy the copyrights to a book in this way? The book was published in 1979 by author John Doe let's say, and then the book was published by Generic Publishing Company. John Doe has been dead for many years. But, is there any problem with me acquiring the rights to the book as an individual?

You will ask them if they own the exclusive copyright and, if so, whether they will sell you the copyright. Their rights may be subject to a royalty agreement with the author that could affect their ability to assign the copyright or include terms that carry over to you. Once you know if they are interested you can start discussing how much they would charge. Then you negotiate based upon your anticipated costs and profits associated with your plan to republish the book and, if you reach a mutually acceptable price point, you have your lawyer provide a contract for the assignment of rights for review by their lawyer.

The publishing company may not have the rights and John Doe may have heirs or other assigns. You're going to have to do some research to figure out the proper person to ask, but the publisher is often a good place to start.

How royalties are computed really depends on just what the book is. Best seller fiction is paid out differently than let's say computer text books. I don't know what you mean by "Generic publisher." If it's the subsidy press it's a completely different than the mainstream. Of course, how book royalties are computed isn't really a legal issue.