" I would like an external filter but as the pump will cost a
fortune "

A submersible pump running to a out of pond DIY filter should not cost you a
fortune. I like azponds.com. In fact right now they have a 2100 gph Laguna
for a $100. A search will give you many DIY filters. The skippy idea is
very common = pond water empties into the bottom of a container and the
'dirt' is trapped as it rises to flow out. The material that traps it also
serves as a medium for the good biobugs. Many would advise against it, but
my filter is similar to a skippy except I use pea gravel instead of
scrubbies. The advantage is it cleans great and is really low in initial
cost. The disadvantage is it clogs up and must be cleaned much more often
than the scrubbies. Jan's demon pond filter is terrific but it aint cheap.
Generally the more you invest in your filter, the less maintenance will be
required.
HTH
Bill