The Power Nazi is back, page-18

Most 2nd hand inverters are not on the approved list anymore which means no RECs (subsidy) for your panels and I would be almost certain that used panels would not qualify for RECs either. There would be no harm in going for a complete used system so long as you could find an installer to certify it for your grid connection and feed in tariff and you were not expecting any government help. All your panels should be the same for maximum efficiency.