Usually 1 or 1.5 feet of pipe per square foot works. The average 1700" house would have about 4 zones at 500' or 6 at 300'. I recall the polyb coming in 300' rolls, so count your zones and typically thats the measure, unless they put joints in the slab. With the brass fittings and no sun on the pipe and no chlorine, it should last the life of the house. I used insert fittings and crimp clamps.
The best water heater I have found is the mor-flo american Polaris at 94% efficient.