As you are a newbie, just a reminder that a nominally 500w motor is illegal to drive on the road unless insured, etc, etc, and illegal to ride off road in any public place (such as bridleways and canal tow paths). In theory as the 500w one makes it a motor vehicle rather than a pedalec you can be fined for driving without insurance etc, and get points on your driving license.
The chances of being caught are very small and of being prosecuted even less; biggest risk is if you have an accident which leads the police to scrutinize the bike.
Most nominally 250w motors actually go to 500w peak, often over 750w, so may well be quite enough for what you need.
For a rear either cassette or freewheel are fine, but if your current wheel has the opposite to the new wheel you'll obviously have the extra expense of a new cassette or freewheel.
If you fit a powerful front one you need to make sure the forks are up to it, and fit torque arms.
