Advice On Adding Gold Fish To My Pond

[KrystaK]

We moved into a new house 2 years ago and it had a beautiful rock fountain-pond combination in the back yard. The pond measured 9 ft long x 3.5 ft wide x 1.5 deep it's ~350 gal if my math is correct. It's a lined hole in the ground with a little bit of sand/gravel substrate (Plus a lot of leaves and debris because we didn't cover it last fall.)
 
It has a pump but no filter. I was told that while a filter isn't necessary it would only benefit the pond. I was wondering if this was an accurate statement? 
What size of filter would you recommend for a pond this size? I assume most pond filters are submersible but still require an outlet.
I won't be putting fish in the pond this year, but hopefully next spring
I was wondering how to cycle a pond? I have a jug of ammonia but I don't know the amount I'd need to properly cycle this size of pond. I was thinking a fish in cycle would be easier, but I'm aware of how harsh that can be on fish.
How many gold fish would be suitable for a pond this size? I've seen that even common gold fish can get pretty big, so I was thinking of something like 30-50 gallons per fish would be suitable, but I thought I'd ask. 
 

 

[Brilly91]

Most pond filters require a pump that takes water from the pond to an external filter, most people use a box type filter which usually contains the normal sponge, bio ball combination.
 
You could keep a good few goldfish in there, id personally go with 10 and see how it feels and add more if things seem fine.
 
My only concern is in the colder months, depending how cold it gets where you are 1.5ft of depth could quite easily freeze.

 

[KrystaK]

I was thinking for the colder months to get a heater. Water freezes at what, 32F? I read that gold fish can hibernate, and I was thinking of getting a heater to keep the water just above freezing, but cold enough to slow down their metabolisms so that I wouldn't have to do water changes in the middle of winter. I'm not sure it'd work, but that's why I'm on here
 
As for the filter, I was thinking of something like this: (I hope the url shows up)
 http/www.bigalspets.ca/powerflo-max-biological-underwater-filter.html