Hi Adrian, where you are plays a large part in the decisions, as does the pond type.
Your gallonage for that size pond is exactly 1500ukg but the time the message was posted makes me think not in the uk.
Here in the uk much of the cost of running a pond of this size can go on heating it, unless you can have it indoors in a heated room. If heating is not an issue then for catfish, bass etc then you will want reasonable turnover , somewhere towards the 12000 l/hr which can mean a pump of 100-£150. That said you need to get an eco model which runs around 100w max - any more and the cost of running it will be > £1.50 per day, which whilst it doesn’t seem much soon adds up.
If the pond is above ground then it comes down to where you have space for the filter. A koi pond box filter can be mounted above the pond or alongside the pond If there is room, and for this the bigger the better. I run a 4x2x2 koi pond box trickle filter on 1200 ukg with 12000 l/hr from an aquaEco-12000 pond pump. Filtration can of course be home made using the same principles if you have a barrel or container and some diy knowledge/ability and this way is often cheaper and can be designed how you want it. Can’t give you all the ways of doing it but at least you can see one way that works for me.