Before you do anything else, you need to read up on M&A. Buy some books. A lot is on the Web. Some of the things you should know about off the top of my head (to get a sense whether you are reading the right things or to help you search for stuff): earn-outs, holdbacks, escrow, non-competes, indemnification, capital gains taxes, stock vs asset transactions, arbitration clauses.
That being said, there are numerous ways to value a company, e.g. multiples of revenue, users/customers, traffic, assets, etc. What makes sense is very case-by-case and industry specific. But what it comes down to is that valuation is just a tool to make a case for deal terms. In the end, a deal is an agreement between two parties about what the terms of the deal are worth to them.
They probably already have an idea in mind about the minimum they will pay, and they are probably trying to draw an initial # out of you. I'm not going to tell you how to negotiate, but just be very wary of this, i.e. throwing out an initial #. It can set the stage for the whole transaction, and since you obviously don't already have a well thought out valuation, you may come to regret whatever you throw out.
Whether to work for them or not and how that would look like, is again, very case by case. For example, it could be structured in a way where if certain targets are met you get more money (earn-outs), or it could be a short transition consulting contract, or simply a salaried position not otherwise tied to the deal. I personally would make sure whatever you get up front can not be influenced by your behavior as an employee, i.e. are completely independent.