1) When determining your zoning, pay attention to floor coverings. Tiled areas will radiat more effeciently than carpeted area. This can sometimes play a larger roll than exposure and window area.
2) Make sure you (or your contractor) has addressed low return water to your boilers. Some boilers have addressed this internally (Weil Mclain Gold series), while some boilers may require external controls.
3) Make sure you address the fact that your indirect water heater will require hotter water (180-200 F), than your radiant zones (90-120 F).
4) Indirect water heaters tend to recover a little slower than conventional gas water heaters, so I would recomend up sizing the capacity, if marginal.