What budget?

If we're not talking fancy (expensive) bikes with adjustable posts, then I would make sure it's something with a round seatpost, so you can find an aftermarket replacement easily - an offset post that can be reversed. There are OK carbon ones out of China, or 3T do some with fully rotatable heads.

Personally I find swapping seatposts easier than adjusting / flipping the same one around - it allows you to use a tri specific saddle for that and a road saddle in road mode. Mark the post height with some tape and it's a 30 second swap over. Something like an ISM adds a lot of effective seatpost angle too, as you sit right on the front of it.

Front end, road bikes are nearly always too high for a good aero bar position, but that can largely be solved with a MTB stem upside down. Something longish will stop you from being too cramped up. The other thing to watch for is clip on bars that are too high above the base bar.

Trek has the new Madone 9.0 that has regular bar set up so you can add aerobars. This could be a very interesting option.

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http://www.TriScottsdale.org

I would look at the AR, S5, and Madone 9.0. Then go look at the PlanetX aero road/stem/Bolt on Tri bar system.

The aero bars skew into the road bar just like a dedicated riser bar, so you don’t worry about the bars slipping on every bump. Nice aero stem that integrates with the bars. They also make a TT basebar that you can swap if you want to go full TT. This would be a sweat system with ETap.

Your choice between the AR/S5 and Madone should depend on whether you can get the saddle in a good position with or without the reversible seat post. AR is definitely cheapest option and it tests very well against the S5. Old S5 has a very tall head tube that could completely ruin a decent aero setup if you need lower.

My dream would be to pair my S5 with e-tap and the Morph tech bars.