Ditto, aero kit -- jersey, at a minimum -- and helmet offer most bang for the buck in getting aero. Next to getting and holding an aero position, which is free but uncomfortable. So plan on burning some energy strengthening your core, and tweaking your bike fit, to hold an aero position on any bike.
The best buys in aero bikes are usually used triathlon bikes. Most tri-folks are primarily runners, who later add a bike just to participate in triathlons. I was skeptical of these stereotypes until I saw it for myself. (I'm primarily a cyclist, secondarily a runner.) Tri-folk who are primarily runners tend to be in one of two categories:
- Ride their bikes into the ground, rarely maintaining them. My 1993 Trek 5900 OCLV was converted to a tri-bike. It was in remarkably good shape when I bought it from a BF member and friend a couple of years ago (he wasn't the original owner and wasn't responsible for the bike's condition). But the headset needed to be replaced within a year. When I disassembled it I found crusty tan residue inside the entire steerer tube, headset, etc. That's almost certainly due to years of the tri-guy leaning across the headset on the aero bars, sweating down the headset, and drooling water with electrolytes (very salty) via a drinking tube mounted on the handlebar/stem. A year or so ago, a friend gave me an old tri-bike with a cracked frame around the seat tube, but mostly very good components worth cannibalizing for another bike. The tri-bike's rear hub was in the same condition -- crusty brown residue, shot cartridge bearings, etc. That's probably from urine. Yeah, some long distance cyclists will just let it flow during a ride without stopping. I didn't check the bottom bracket but it's probably in the same shape.
- Or, if they're prosperous dentists or lawyers (yeah, I know, stereotyping again), they'll buy a new tri-bike every couple of seasons, believing it will make them significantly faster. The latter can make for good buys in used but not abused bikes.
Tri-bikes can be ridden reasonable distances with just the bullhorn bars, if you're not comfortable with aero bars. And you can fit them with conventional drop bars. While not ideal, they're okay for rides of an hour or so. Not bad for chasing PRs. (At my age KOMs are out of range. If a 63 y/o dude in middling shape snags a KOM, you can bet that segment has only been ridden by moms on Townie cruisers and kids on Big Wheels. No 60something cyclist will ever hold a KOM if any actual cyclists ride that same route. Even with aero help, I lost all my Strava Top Tens a year or so ago as those routes were ridden by younger, stronger riders. Our local women's pros all beat my best times.)
Don't count on finding similarly great buys in recently used aero road bikes. Roadies tend to expect more reasonable market value for their bikes, which are usually in better shape.