Sounds good. If you can't fit your tanks inside your greenhouse it's o.k to have the tanks outside the greenhouse, and just cut a hole through the plastic wall for your piping. Fish are more adaptable than plants, and that might give you more room. If you use a cold water species like trout they will do fine in freezing water, just cover for snow.
I'd start with a 1000l tank, or even smaller. How big is your grow bed?
Our first aquaponic system was literally a tomato plant floating without soil, in a small 2ft terracotta garden pot filled with water, with a couple of goldfish in it
That system taught us that piping leaks are more common than you think, our tanks spring a leak, usually from a burst pipe, a couple of times a year. You learn little tricks, like placing your pump on a couple of bricks, to ensure that even if the tank fully drains, there will always be a puddle of water at the bottom to keep your fish alive until you discover the leak, but better to discover a small puddle than a flooded greenhouse. As you increase the size of your tanks, you'll still make mistakes, only they get bigger
How much room have you got?