Pam, a practical tip from someone who's been there...
When I de-cluttered my life in 2005, I sold my koi for a ridiculously low price. Not free, but 1/15th of what they would be worth. That way you know it's folks who really want them.
If they are serious they will come over, catch them, put them in plastic bags with oxygen, and at least you'll get a bit of money. (!)
Best would be to join a koi forum (local or not), post a pic of pond/fish, explain you need to move/whatever other reason you want to put, and put a low price or ask for a best bid. There will be serious enthusiasts - often people who just had to rebuild their pond or had a calamity/disease that wiped out their population, and would JUMP at such an opportunity.
That was what happened in my case; a nice couple from Amsterdam came all the way with a van, big plastic bags, a baby washing tub (I know...) to make it easier to put them in the bags with a bit of water, and a big oxygen tank so they would not suffer on the 2 hour trip back. (oh, and yes, they all made it without issues and most are still alive even today)
FWIW I sold them for $1000 - they were worth about 15 to 20k. All depends on how many, how they look etc, but if you can catch say 500 quid, why not?!
Pond will not really smell all that much
normally if it has been filtered and cleaned properly. Even if it would, after a few rains that will be gone.
A small pond is still a lot of work. Think carefully if you want that. In fact, small ponds get dirtier quicker (think about it! less water volume, so every bit of "debris" takes up more space percentage wise). I would advise letting it fill.
Ask around for a construction company who needs to unload their stuff. Doesn't really matter if there are rocks in the sand, right? I'm sure you can get that done for free. Put a small layer of top soil, some grass or whatever over it: done.
See pics to give you an idea.
PS: I did never fill in the pond though. Just kept small goldfish in them, no filtering. In the last 5 years I started again, but small, 5 big guys max. And yeah, that still takes 2 hours/week to properly maintain/debug etc. So in your case you should probably go the whole way.
PS2: last pic are the buyers, like I said, great folks, they were happy as can be!