Strength for this example will be very important. The hand cart needs to be able to withstand a certain force, especially when being dropped off a curb. The two plates (back and nose) will need to be strong enough to withstand the 300 lb force, as will the connections of the plates (bolts or welding). The axles for the wheels will also need to be strong enough to withstand a jolt from being dropped off a curb.
Some strong materials you might consider for the plates and the axles are:
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