Robust, MECE but simple M&A Framework

You never want to use a pre-thought out framework in a case, so I would throw out any notions of having anything ready. What you'll find is the case will be laid out in a way which means that you don't have to ask questions about certain areas, but which will invite you to dig into more.

In terms of M&A, it's generally useful to think about:
The market - Is it the same, is it different? Customers/ geographies? Is it an attractive market? (large, profitable, growing?)
The target company - Is it successful? (Growing, profitable, strong positions, unique capabilities, etc.)
The purchasing company - Should they be making acquisitions right now? Can they afford them? Is this the right market to be making an acquisition is? Is this the best company?
Synergies - What is the increase in value from the merger? Cost synergies (HQs, procurement, etc.) revenue synergies (customers, marketing, etc.) and what about from how the company is currently being run?

It's very unlikely you'll ever get a case which asks you to cover all of the above, as it's simply too much and won't allow you to get into the detail. More likely it will be a deep dive into one area - for example, answering the question of whether the market is attractive in more detail.