This questionnaire should help you identify your requirements as you evaluate products.
Which operating system(s) will you use to host the service?
Which programming language(s) will you use to write the service?
Will you need an adapter to connect to a legacy application?
Which Web server(s) will you use?
Which application server(s) will you use?
Which database system(s) will you use?
Which user management system(s) do you use?
Which access control system(s) do you use?
Which management infrastructure(s) do you use?
Will you support internal or external clients (or both)?
Is there a chance that you will add new clients over time?
Will you have control over the client ...