So my company is looking at doing some work through some sort of share site/collaborative tool.

We'd need to be able to keep control of our forums, even if we gave people from outside the company access.  It needs to be possible to upload files and have others comment on (and ideally edit) them.  Ideally there would be the ability to have multiple forums, permission levels, projects/files etc.

It can cost money (within reason) and needs to be secure. 

As I am far from an expert, I would also appreciate any advice on what other features I should be looking for?

Cheers.

Mustn't be anyone's area of expertise

you might want to try an actual IT forum

Without knowing the situation in detail I don't think I know enough to suggest anything, an intranet system of some description could work but I am not sure, I have no real experience with setting one up or supporting one.

Try googling Drupal it was what I used when I did a project on one once, it's open source so free to use from memory. It supports forums, password protected access, a job system, calendars/events and file sharing IIRC.

Can you recommend a good IT forum?  That is a good idea!

can't think of a good one for this sort of thing off the top of my head but google is your freind, any would be better than this site for these type of questions.

Only thing I could think of is Squarespace and that isn't exactly for forums...

Ask at at one of the derivatives of Stack Overflow.