I didn't want to buy tri gear before mine, either, since I didn't know if I planned to make a habit of it.
I just picked a couple of pairs of cycling shorts that seemed likely (thin chamois, substantial lycra) and got in the shower with them on, to see how they'd do when they were soaking wet.
Whether the chamois would take on an unacceptable amount of water, whether the lycra would start to sag. The pair I picked was absolutely fine.
I did the whole thing in just a sports bra/top. If you don't have one that's a style/color you feel comfortable wearing without a shirt, that's something you could get that isn't really tri specific. You can always put a shirt on at T1 if you're uncomfortable riding with your back and belly exposed. A pair of shorts with a higher rise might make you more comfortable that way, too.
Speed comes from what you put behind you. - Judi Ketteler