Water Heaters - Adding a recirculating pump on tankless heater

We need to replace an old Sea Horse tankless water heater. We selected the Bosch Aquastar 250sxo (must be installed outside). The heater will supply hot water to an Apollo Hydroheat air handler as well as hot water to the rest of the house fixtures. Our system also has a ~30 gallon buffer tank. The old water heater had a built in recirculating pump that recirc'd the water between the tankless heater and the buffer tank. The pump and tankless heater was triggered by a Honeywell Aquastat that measured the temperature in the buffer tank. The new Bosch/Aquastar does not have a built in recirculation pump. Can someone suggest a pump that would be appropriate for this application? Size, specs and maybe a brand or vendor? The pump also needs to be installed outside and it appears that there is not enough room in the case of the Aquastar, so it will be exposed to the elements.
The original installer of this system is no longer in business, so we are left to DIY! Any suggestions or references would be most appreciated.
Chris T.
Morehead City, NC

Most recirc pumps can't deliver the volume or pressure required by tankless heaters, but Metlund makes a couple of models that they say are approved. However, the Metlund pump is designed to be installed at the point of use, unless you have a dedicated recirc line, which you might have if you have a holding tank. But neither the 250SXO water heater nor pump are designed for sub-freezing temperatures.