You don't need to drain the oil, but there will be some in the filter housing. So when you remove the sender, you will get some oil draining out. At most, it will be about 1/2 a quart.
The sender is only accesable from under the engine. You will need to remove the lower engine cover.
The enitre job should not take you more than 1 hr. Once under the car, you can easily see the sender attached to the filter housing.