You might want to start small and work your
way up. See if you get bit by the pond bug and
then go for koi and really give them all that they
need.

You can start with 180 gallons but put in three or
four goldfish. You can filter the pond with plants
(this only works with few fish to much water
and many plants)
in the pond and run a pump set up with a fountain.
That's the simplest and cheapest way to start a pond.
Then you can get a feel for it, save money, read a lot
and build something bigger for koi. As you saw they can
get really big!

I have goldfish and a couple of koi in 3,000 gallons. I filter
the pond thru watercress in the waterfall and water hyacinth
in the waterfall pond once May comes (when it becomes warm
enough for this tropical plant).

If you are handy there are many do it yourself filter plans out
there (many rec.ponders have them) and build a filter to handle
a larger pond and bigger fish load.

You will get lots of good information here. You can pick and
choose whatever feels right for your situation.

kathy :-)