hooking up a outdoor boiler to a open direct system

I purchased a new AFS900. Here is my current system. I have 1400 sqft of radiant floor heat that is heated by bock oil fired hot water heater. The system is a open system (domestic water is radiant water.) I also have a garage that is heated by a hotair furnace. My plans are to install plate exchanger on the cold in on the bock heater and install a air exchanger on the hot air furnace. I have a couple of issues. What size Plate Exchanger? Should I install a mixing valve for my domestic water 135 degrees or can I run my Woodmaster at a lower temp and still run the air exchanger in garage?