My advise is not to scrimp. We had ours renewed a couple of years ago and what a difference it made. Previous to this we had just two solar panels and and we also had an emersion heater.
When we renewed we got three new solar panels. Since this we have not had to use the water heater at all. The water stays hot much longer.
Neither of us can remember how much it cost. I seem to have a figure of 1,000TL to renew, but not really sure.
There are quite a few places on the Gunlukbasi road that sell these systems.