Well, I have spent considerable time researching this and will continue to do so.
But, I am somewhat confident in the following two companies and their air handlers.
These air handlers are designed specifically
for hydro-air systems, whereas other units made by Carrier or Lennox for example, are not.
This link is broken, either the page no longer exists or there is some other issue like a typo.http://www.summeraire.com/airhandlers.htm
The Summeraire air handlers are also "available" with a ECM motor making it much more energy efficient.
ECM motors are increasingly being found in forced-air furnaces and HVAC systems to save on electricity costs as modern HVAC systems are running their fans for longer periods of time (duty cycle). The cost of using ECM motors in HVAC systems is higher, but the major benefit is the motor efficiency is much higher that other motor types at reduced speeds and partial loads.