Ubuntu server with subversion. Ubuntu is one of the better stable flavors of *nix and better ability to do apt-get and keep applications up to date. Subversion works the best with the ability to do hooks.
Ubuntu+Mercurial.
I use Mercurial+ssh for private repositories and bitbucket for public repos.
My server for a one-man project (just me) is running OpenSuSE 10.x. It hosts a number of subversion source repositories.
OS and repository preferences are highly subject to personal preference. I use OpenSuSE and Subversion, and they work for me. I have used many linux distros, dating back to Slackware version zero-point-eight, believe it or not, and I like a lot of them. I've used mahy repostories, including CVS, visual sourcesafe, clearcase, and subversion. CVS is rock-solid and smokin'fast. Subversion is fine, but isn't as solid or as fast as CVS, but it's good enough for my purposes.
Well I use Fedora for this purpose, personally. Reasons being that it is a branch off Red Hat, so experience with Fedora should help me if I ever switch to development on Red Hat (a common server choice).
It sounds like this is a startup like scenario and not within an existing organization. Is there any reason not to use a hosted solution? You should focus on your core competencies and not on administering an extra server if you don't need to. XP-dev.com offers free and private 1.5GB subversion hosting. Github offers reasonable $12/month plans for private repositories.
Say you lowball value your time at $25/hour. If you spend more than 1/2 hour per month either administering your server or having wonky problems that hamper your productivity, then it would be more effective to just buy the github account.
I think that you'll find Cent OS to be a good fit for your purposes. It takes RHEL (which is focused on stability) and removes the branding in order to make it free, OSS again.
I don't think that you really asked for a version control recommendation but I'll talk about that too.
I have a dedicated Ubuntu server using GIT for version control.
Managing private repositories with gitosis. And it's been great.
Manage git repositories, provide access to them over SSH, with tight access control and not needing shell accounts.
gitosis aims to make hosting git repos easier and safer. It manages multiple repositories under one user account, using SSH keys to identify users. End users do not need shell accounts on the server, they will talk to one shared account that will not let them run arbitrary commands.
It's robust, damn fast, and reliable.