My data is a 'mess' and too complex to define, I need to conduct detailed analysis before commencing?

This case is exactly where 1View succeeds. It is the tool that allows you to discover and manage data, irrespective of its initial quality, and to adapt it to change as your data changes.

Is there a limit on the number of files/rows that can be processed or the number of users?

No

What types of input files can be catered for?

Any tabular data of fixed width format e.g. csv, txt, xls.
Any complex layouts can be catered for by 'add-ons' if necessary.

How can you match if there is not a field on both files which can be used as a common identifier?

Sometimes a combination of fields, or a 3rd file can be used to achieve the necessary cross reference.