Purely FWIW...
RE: Thunderbird... I'm a big fan of Firefox, but fear for its longevity -- things aren't going extremely well in Mozilla land. IF that sort of thing bothers you it *might* tip your decision towards Microsoft's Live.
RE: Performance... I still have XP Pro on this rig, along with 7 & 8.1. Running the same software, 8.1 can be marginally faster, I assume because of better memory handling but could of course be wrong. XP Pro is every bit as fast as 7 as long as the task doesn't need the full 8GB of RAM.