1) Don't plumb it backwards!
2) Make sure you leave enough space around the evaporator to draw air. Most manufacturers recommend 2' of space.
3) Leave at least 3' in front of the heater for servicing the unit.
4) Make sure that if there are any overhangs or eaves overhead, you have at least 5' of clearance.
5) Make sure there is a quick disconnect box close to the heat pump location.
6) Make sure your electrician uses the correct amperage, breaker, and wire gauge.
Good luck with it.