Help - New Large Koi Pond

We have a large spring fed pond on our property - 45x55, that we are
trying to make into a koi pond. It has a couple of trees around it but
mostly sunny area. We are getting conflicting advise, from the need
for filters, liners, caves, ledges, plants vs no plants, etc to being
told just circulate the water and throw in the fish. Where can we get
good advise - most of the books seem to be for much smaller ponds. We
are in Upper Makefield PA, 35 mi north of Philly Thanks
- Jeff