SCM for multiple projects + distributed team

In our organization we have 6 teams: 3 "project" teams who provide software to the end user 3 "supplier" teams who provide component software to the project teams

I am the leader for one of the supplier teams. I manage code which is standard across the 3 project teams, yet configurable for the 3 project teams. The code is divided into "static" and "config" code. The static code is meant to be the same across the 3 projects; the config code is meant to be different in each project.

I have to orchestrate a rather large team to make my 1 supplier team's deliverables. We are a distributed team, with 6 developers on-site, and 4 different contracting houses providing code which contributes to my "mainline". Each of the 5 sub-teams contributes to a different part of the mainline, but we are allowed to touch each other's code. Total around 35 developers.

What would be the best SCM tool to use, and how would you set it u开发者_开发技巧p? Constraints: no budget; unwillingness from management to accept the risk of introducing a new tool. In short, the current in-house team of 6 must continue to use VSS, while I (the build master) might have the ability to buy a tool (1 license only) to help me manage the flux of code to/from the contracting houses.

It looks like Accurev might be a strong contender but I wanted to poll the community. Whatever tool I buy would have to support (somehow) merging code into VSS and being able to provide (nightly?) builds to give to the other teams off-site.

"distributed teams" means DVCS, Mercurial or Git.
Don't even try to manage code/patches with a CVCS (Centralized VCS) amongst distributed contributors, you would only end up re-inventing what a DVCS does naturally anyway.

If you really need to maintain local VSS repos, you can try git-vss in order to facilitate import-export.