If you want to make some money back, you need to find something that is in high demand, but low supply. Best of luck with that.
Guppies will sell if you get a rare colour, which is only rare until you sell them. We got a beautiful tuxedo red, when all the petshops had yellows and blues. Now you cant find a shop that isn't selling exactly what I thought was rare. But by the time I've got blues breeding, they'll be back in the shops.
Some really nice fish are the special types of pleco's, but some of these have a lot of difficulty breeding in captivity, and they're not cheap to start with.
However, I really do like the fishroom idea.
Heating - panel heaters are good for taking the edge off a room, for true heating, use a heatpump. Make sure the room is well insulated.
With the sump idea - If you must have one sump shared between tanks (and for a fishroom - you probably will), I would try to have multiple sumps so that it's only 3 or 4 tanks sharing each sump. It's more expensive, but it's better than 7 or 8 tanks sharing a sump.
Grow out tanks - With eggs spawning every two weeks, I would like to have at least two grow out tanks per spawning tank, so I can even out the growth and keep a close eye on them.
Some plants can help with filtration, and they help some fish de-stress by providing hiding spots.
As for the size of each tank, I personally believe that anything under 60L is cruel. The bigger the tank, the bigger you buffer is for dealing with spikes.
With the aging water, I would keep this as independent of the rest of the system as possible, because if something goes wrong with your water supply, it could be a massive issue to try and disconnect it to sort it out.
It may also pay to think of a failsafe for the water removal to your gardens... You don't want to forget that you left it draining and come back to find the fish in the yard... Most people would keep an eye on it, but sometimes things come up.
Best of luck, hope it goes well. Please take lots of photos for us.