I really like Confluence for collaboration. If you are only a few people then there is a 10 user license for $10(it gets quite expensive after that). Zimbra is rather easy to setup and use so I would not count it out just because it has more features then you need.
If you are looking for something simple then have a look at Advanced Client Portal on code canyon, it is only $20 and does what you say but beware if you are uploading large files you will have to edit your php.ini file to allow for large uploads and depending on your server there may be restrictions to doing this. You could also use google for business but it sounds like you are looking for something really simple and the Advanced Client Portal is perfect for this as it allows messages, project management and even invoiceing so it is a pretty tight package and looks quite nice as well.