Is this an accurate summary of why you need a system?

We can't find information we need

because

There is no single repository of information to refer to

because

There are lots of emails and documents with scattered information

so

We need a system to centralise information from emails and documents

If the answer was yes, I understand how you got to the "I need a system for this" solution.

But what if we keep going?...

We can't find information we need

because

There is no single repository of information to refer to

because

There are lots of emails and documents with scattered information

because

There are frequent changes in requirements

because

Customers aren't yet sure what they want

Why aren't customers sure what they want? There is another layer of "because" that you need to get to. Solving it might mean the problem of constant change and the associated emails and document revisions goes away.

Is this an accurate summary of why you need a system?

We can't find information we need

because

There is no single repository of information to refer to

because

There are lots of emails and documents with scattered information

so

We need a system to centralise information from emails and documents

If the answer was yes, I understand how you got to the "I need a system for this" solution.

But what if we keep going?...

We can't find information we need

because

There is no single repository of information to refer to

because

There are lots of emails and documents with scattered information

because

There are frequent changes in requirements

because

Customers aren't yet sure what they want 

so

I need to help my customer develop their wishes into something I can work on.

Even then, there is probably another layer (or more) of "because" that you need to get to. Solving it might mean the problem of constant change and the associated emails and document revisions goes away.