How do I evaluate the legacy system environment?

Factor	Questions
Stability of supplier	Does the supplier still exist? Is the supplier financially stable? Can the systems be maintained by anyone else?
Failure rate	Does the hardware have a high rate of reported failures? Does the software crash?
Length of service	How old is the hardware? How old is the software?
Performance	Is the system performance adequate?
Support requirements	What local support is required? Are specialist staff skills required?
Maintenance costs	What are the maintenance costs?
Interoperability	Is the system difficult to interface with other systems?

Reference: Software Engineering, Sixth Edition, Ian Sommerville, Addison Wesley, ISBN 0-201-39815-X