Ditto what Knot and Flybye have said. You can still be tweaking fit when you do the tri - 15 miles isn't really *that* far (and you've been doing longer distances on your hybrid, no?).

I think a lot of it depends on whether you've decided (at least sort of) what you want, and whether there's a decent chance you'd find that used. I've bought 2 used bikes, and both have been good experiences. Of course, a grain of salt is in order

CA