Used is your answer. You'll want a metal pumping unit with ceramic components versus a plastic setup with plastic parts that can't handle high pressures required for tree spraying or soil injections.
Look for a flow capability of 30 gallons/min and an operating pressure of 300 psi. You likely won't require that PSI very often, and the actual flow is far less, but if you go with less, it'll take forever to get the work done.