Both pumps you have selected will work. Both have the correct inside materials. Flow will be fine with either. I normally use the 813-239 and turn the pressure switch down to about 120-110psi.
The accumulator is used to protect the pump from short off and on that can destroy the armenture. I use one about the size of a soft ball. It is a diaphram type with air pressure on the top side.
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Brad
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