Hi Mike,
Like most processes when you transfer energy you have losses.
Does you present system overheat? if not then it is unlikely that by fitting a heat exchanger you will suffer any problems.
I have been running a homebrew system for several years now.I have used an old domestic copper hot water tank with internal coil, as a preheat tank before going into the main boiler tank.
It is a thermal syphoning system so no pumps, which is filled with plumbers antifreeze.
The collector is an old house radiator suitable boxed etc.
It is an unpressurised system with small header tank and vent pipes.
The system does not produce much heat in the winter, just raising the temperature a couple of degrees. With a pump, a better collector and pressurised system this would be a different story.
If your solar tank is an open top you could just coil up some copper tubing and drop it in. Do not forget a filler pipe and vent.
You could probably pick an old domestic tank up from a local plumber cheap. A lot of people here in the UK are going to on demand boilers, so the plumbers are ripping the old tanks out and take them down to the local junk yard for the copper value.