One has to be very cautious while selecting the solar water heater. Many variants are available in the market with wide price range. The main two basic types are ETC(evacuated glass tube) & FPC(flat plate collector). ETC is an economy model. IN ETC systems the absorber being glass tube it is fragile & susceptible for breakage when exposed to high temperature, particularly in summer. One tube breaks, if not plugged immediately, terrace will be flooded until the over head tank goes empty. Off course the catch is price. The tempting price masks the disadvantages which demands caution.
If you are looking for a long lasting maintenance free system FPC is the best bet. It can cater to your all types of needs. Keep in mind following while deciding on your requirement; if you have corporation / Cauvery water - go for the basic system & if you have connected pressure pump go for Pressurized system.
if it is bore well water with TDS > 250 ppm - go for heat exchange model, if you have connected pressure pump, go for Pressurized heat exchange model. Tanks with suitable protective coating & jacket type heat exchange are the best bet to handle all your water hardness issues.
Check on manufacturers capability to meet your requirements & ability to provide after sales service. There are systems working in the field since 10 to 15 years!