filter help

I am helping my gal friend on her pond. A previous friend did all the labor but was unable to figure out the next step. Currently she has around 6 med sized gold fish in a 750 gal (approx) pond. She is going to sell her home so we wanted not to go cheap, but inexpensive. found a few on craigs list but not sure thats what we wanted. I need to know, a gravivity type of filter seems cheeper, is it because it needs to stay close to the top of the waterfall? I have seen little giants , tetra and fishmate. i already have a pump that is rated at 3000 gph. i can pick a filter up for 49.99 to 99.99. I like the fishmate but is a gravity feed. does the pump have enough push to get water volume up to waterfall. #1/2 feet from surface. Just looking for some advice.
Mike

Gravity fed are harder to keep clean, at least I believe so.

The cheapest option it so build your own filter. It isn't very difficult , you just need to decide what kind of filter you want. You can turn a free plastic 55 gallon drum into an excellent filter, or even a 20 gallon plastic tote.

If they are looking at selling, you may want to buy a filter then. I can not help you there, as I have only made my own filters.

Look at the bigger picture. A house sale is generally huge, so my advice is pay whatever to protect the bigger issue here, the house sale. The longer you labor over this, the longer the house will take to sell.