What about using SALT, that might solve your freezing problems, from my experience and the description of the system you should be fine. If you want the system to last from 5-10 years, you are going to have other problems before the salt corrodes anything. The copper should be fine, however I don't know what the coils are made of so that's the only things i might worry about. Pool pumps are typically plastic and most of them are designed for both fresh and salt water. Anti-freeze is evil! It sounds like you want to help the environment, so stay away from it. Do it the way nature does, There is lots of liquid water in the artic circles- because its salty.

I want to build this to circulate hat/warm water through hose coiled in pots to keep a small number of plants extra happy through our mild winter (high 30's - 60s during winter) If I build it this way, it will be closed loop and does not matter if the water is contaminated I guess...still, needs to be cleaned somehow.

Suzieque - Each box is measured differently and you need to examine the "installation/specs" guide on line for every one you are considering to get the actual depth, plus add in the clearance to find out exactly how "counter depth" your fridge will end up. Usually the doors have to project beyond the counter to clear the hinge centers, but even so the total depths can vary depending upon manufacturer. This was very frustrating and time consuming for me since I unfortunately had just under 24" of depth to work with on an outside wall and really wanted a built-in appearance due to my very tight space. I found that so many units were over 28" deep when adding in rear clearance, and that each unit had a specific requirement for plug location and waterline hookup to really maximize the look. One or two more inches of play would have truly helped so much but alas I had to use every trick to make it happen, from recessing the plug just-so, to recessing the waterline and angling the connection to allow the copper line to coil up flush, etc, etc... After picking the refrigerator I thought was truly the best for me( imagine going to the dealer and rolling each box out of its display area and measuring to the very back with a tape to find the Actual depth...) That said, I'm very happy with the results- looks nearly built in, but you do have to check each candidate carefully to get the actual dimensions and then add in the recommended clearances. BTW- some manufacturers do seem to "approximate" the numbers slightly, so do check yourself to avoid surprises later... and salespeople mostly tell you that they are all the same, but it's not true...

spray the coils with non-acid based coil cleaner, let it soak in for a few mintues, then rinse and repeat if needed. the water and cleaner goes out the drain. you should see 1 drain line coming off teh housing itself, then a second back up drain line coming off teh drip pan below the housing. both of these should be kept cleaned out, pour some bleach down them 1-2x a year. if your unit has the filter in the airhandler, then pour some bleach down the drains each time you are up there to change the filter. BTW, if the coil is extremely nasty you may require a pro to do the cleaning. they will have to evacuate the system, remvoe the coil, clean it, then reinstall and recharge it. NOT cheap, but is necessary in many cases.