Evaluating OSS for the enterprise

This is a short answer, because I can only point you in the direction of finding the methods of evaluation. All the work will fall on your shoulders.

First, build a decision matrix listing the qualities or features that you want, or the problems the software must solve. For each one of these qualities, add a value to express how important each item is to you, relative to the others. The resulting numbers can help guide your decision and make you think hard about what you value and how much you value it.

Maturity modeling is a long-established engineering/quality-control method of assessing a technology, and has been brought into the software world. Bernard Golden's book, Succeeding with Open Source, gives an introduction to this method and shows how to apply it to open source software.