My first pond had an internal pump, lets just say that my second does NOT. Internal pumps can be BAD news! Nothing worse then coming home and hearing your pump sucking air ... and noticing no water in your pond. If it is going to be internal, make sure you have the pump inlet high enough so you don't end up draining your pond if your plumbing breaks, or a filter gets clogged.
I think I can speak for a lot of ponders here. The advice given is all from experience. I'd never build a pond without a bottom drain, preferably running into a settling chamber. A skimmer and a decent sized bio filter is also a must.
My pond is almost completely maintenance free with crystal free water. I haven't touched my bio filter since I built my pond 2 years ago. The bottom drain gets the leaves and anything else that sinks before the skimmer gets it and it ends up settling in my settling chamber. I don't have any filter media to change or clean.
My old pond had a filtered bio filter. I'd have to clean the media once a week, if not more often. The setup I had was just too much work for me. Too much maintenance and the pond just isn't enjoyable anymore.
Either way you go, you'll enjoy your pond.
I was not keen on this very outcome but wanted the pump on the bottom so made up a float switch that is submerged (and hidden) to a level to stop the pump very early in case of a plumbing leak before a lot of water is lost.