Do a few and see if you like it; used bike ads are littered with tri bikes of people who disovered they didn't quite like the sport, or doing an IM was their bucket list sort of thing.
I've done some triathlons (half and full IM, too) on a aero road bike without aerobars - I just couldn't get on with them in the short time I tried it and I didn't want to massacre the fit in order to make the aerobars really work, either.
So, you can do half-IM and full-IM on a normal, UCI legal, perfectly ordinary road bike. You can even do it reasonably well. Still, there is a difference and if you end up liking the sport, you are really best off buying a tri bike and riding on it a lot to get accustomed to developing power in the position. Looking at guys with similar power and similar weight and height (it helps to be thoroughly average on all counts, heh) on tri bikes on the same races which weren't exactly flat, about 10-15 minutes on a full IM distance. Mind you, I'm pretty aero for a guy on a road bike - I wear size S trisuits (that's XS in US sizing), aero bike, 45mm deep wheels with a 23mm tire in front, 36cm handlebars slammed all the way down and I can ride in the drops for hours on end, and I really get "nose on the stem" when descending, so we're really talking about a pretty optimistic case for a road bike. Yet, to illustrate the difference in aero drag, doing a hilly half-IM last fall on stretches of downhill where it was impossible to pedal I reached a max of 79 km/hr at the fastest point and the guys of similar weight and height on tri bikes reached 85-89 km/hr. If you're trying to be competitive about it, and if you road raced you probably are, you lose a significant - in terms of race results - chunk of time by using a road bike.
I started riding indoors on my wife's tri bike recently and you really have to do it to get used to developing power in that position because it feels way way harder at the start. I can now totally understand a few long distance triathletes I know who just ride their tri bike all the time. Maybe do a triathlon or two on whatever bike you've got, but if you decide to really do triathlon, best off buying a tri bike (do as I say, don't do as I do!).
Mind you, draft-legal (typically sprint and olympic) a road bike is required and only shorty clip-ons are allowed, if you're going to do that you can't use a tri bike anyway.
Last edited by Branko D; 01-26-21 at 07:58 AM.