^if you feel you are confident to suggest finishing in the top 100 of a first time event then I don't see why you would question purchasing a tri / road bike brand new? It sounds as if your mind is made up as if this is something you want to do, so have at it.
Since you have some monetary constraints, I'd even go as far as renting a bike just for the race. I mean granted you'll want something you've ridden before but who's to say you can't rent a bike for a few days hell even a week prior to the race. If you must purchase an 80's bike and upgrade parts, I think your money would be better suited towards buying a brand new bike and training on it...save as much as possible.
For me, I'm curious about doing a tri, so instead I did a duathlon (run bike run) with a brand new bike with stock parts on it (crappy low end groupset) purely for the race. My goal was to finish, but now that I've got that under my belt, I'm either upgrading my grouppe or purchasing a new bike.
Good luck and godspeed