Choose ONE use case described on the very first page of this book. Describe what is required for this use case in terms of hardware and software capabilities. Choose products, which provide these capabililties. TEST the products a multi-vendor scenario: hardware from one vendor, device management abstraction software from (ideally) different or (less preferrable) same vendor, service management logic from another vendor. Eliminate alternatives, which give you extra features, which you will not needed for these case.
Choose second, third, ... use case. Follow same pattern.
Choose a set of products, which proved themselves easier to adopt in multi-vendor tests.
Until you really have extra money to spend for fancy features, eliminate other producst, which provide extra features, which you will never use.
Good luck!