In my last Drupal project we were 5 people doing coding and installing new modules, at the same type our client was putting up content. Since we chose to have only one server for simplicity there were times were many people needed to write to the same files like style.css or page.tpl.php or when someones broken code would prevent others from working
Are there any best practises for a team that works with Drupal? How can leverage code repositories or sandboxes?
A single server may appear to give you "simplicity", but what it gives you, as you've experienced, is utter chaos -- and you were lucky if it didn't result in unpleasant and hard-to-reproduce, harder-to-fix crashes. Don't settle for anything less than a "production" server (where your client can be working -- on content only -- if they like minor risks;-) and a "staging" one (where anything from the development team goes to get tested and tried for a while before promotion to development, which is done at a quiet and ideally prearranged time).
Second, use a version control system of some kind. Which one matters less than using one at all: svn is popular and simple, the latest fashion (for excellent reasons) are distributed ones such as hg and git, Microsoft and other have commercial offerings in the field, etc.
The point is, whenever somebody's updating a file, they're doing so on their own client of the VCS. When a coherent set of changes is right, it's pushed to the VCS, and the VCS diagnoses and points out any "conflicts" (places where two developers may have made contradictory changes) so the developer who's currently pushing is responsible for editing the files and fixing the conflicts before their pushes are allowed to go through. Only then are "current versions" allowed to even go on the staging system for more thorough (and ideally automated!-) testing (or, better yet, a "continuous build" system).
Basically, there should be two layers of defense against such conflicts as you observed, and you seem to have deployed neither. They're both essential, though, if forced under duress to pick just one, I guess I'd reluctantly pick the distinction between production and staging servers -- development will still be chaotic (intolerably so compared to the simple solidity of any VCS!) but at least it won't directly hurt the actual serving system;-).