Depends what you mean by DIY. As you will know you are not allowed to do any plumbing unless you are licensed to do so. But . . . doing a retrofit solar HWS is not all that hard - look at some of the kits for retrofit on eBay sites and you can pick up good quality Chinese systems ready to go or racks of tubes with fitting kits to retrofit (Solarvox SolarVox :: Home is one mob and I have used and seen used their stuff and have seen stuff from these blokes 30 Tube Solar Hot Water System Collector Kit include Circulating Pump & PV Panel | eBay an they are OK - there are others).
You need to know what it is you are buying and where so for example, do want a direct or indirect system and whether at mains pressure or not and on the roof or on the ground and so on. Many chinese systems are indirect ie: the solar heats a tank of water in which is embedded a coil of tubing which is a heat exchanger - and the HW comes through that exchanger to your HW taps at mains pressure (although it can be at lower pressure or gravity feed too). A direct system has a mains pressure tank and the water in it is separately circulated through the heating tubes - the stored water is going through the heating tubes.
You can get into strife if you don't understand what is being sold to you (and what you expect & need). Some come with boosters, some don't. If you search this site for solar HWS or solar hot water you should find plenty of info in various threads. The eBay ones often have no subsidies, but are so cheap as to be well worth it anyway IMO. The sizing of the collectors and the tank is critical too.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.