Copper tanks are not a good solution as they are very thin and not designed to be used in a pressurised system. The water here is also extremely high in calcium in some places and iron in others. The tanks here tend to be steel with an enamel coating but they do still suffer.

I've seen several of the gravity type systems totally furred up with lime scale and am not a fan of them. I have a stainless steel one that I acquired from a job and it has been totally eaten through where the joints were made.

I've just been using regular run of the mill central heating circulating pumps in my systems and haven't had any problems with them at all.

Re the thermosyphon - would it work in a coil as you will have hot water at the top and cooler at the bottom of each coil?  A better layout would be a continuous S shape so the hot water would rise and pull the cooler into the tube at the bottom.  Beware that if it is in a box it will get hot and if it is under any sort of pressure then the plastic may burst. If it is a closed circuit then it will develop pressure and you will need a pressure vessel but this may produce more pressure than the pipe is capable of sustaining.

If you have an open system with the 1000l then this isn't a problem but then your hot water will be at the pressure related to the height you locate your cube at which, unless it is on the roof of a tall building, is unlikely to be enough to fire a gas water heater should you decide to put one in line as a backup (they normally need a minimum of 0.5 bar to fire and more to get a decent flame for heating). It will also take an awful lot of sun to heat up 1000L of water!

If you use a pump then because of the extremely poor conductivity of heat through plastic don't expect to get a very high temperature increase. You would be much better off using copper pipe however for 1000L you would need an extremely large panel - or panels. One large panel would probably end up with the water boiling in it so you would need more than one to achieve the desired result. If it was a closed system you could pressurise it to avoid boiling (look up Boyles law) but then you are definitely going to need copper.

Ah, it's a minefield.. but don't be put off and don't take my word for it - if you have time then try it out, the learning is as much fun as the doing and if it works you'll have the satisfaction of knowing that you built it.  :)