Rules are simple IMO - go with a local business which has been doing it for a while, go with word of mouth from people who are happy with the work done on their systems, get 2 or three quotes.
Make sure that you are not asked to pay more than a 10-15% deposit and do not pay any final amount until your system is connected and working and all the relevant approvals are in place (especially the grid connection and whatever the arrangement is with your electricity supplier).
This is an investment like anything else - so you will be spending cash and converting it to an income earning asset. Depending on your age and investment return requirements PV can be a good investment. If your aim is to help the environment then buy 100% GreenPower from your utility and invest your money elsewhere.
It will take between 2-3 years to get payback on a 3kW system - or more depending upon what your are able to get for the power you produce (it might be much longer). Then to get equal to where you would have been by simply bunging your money in a fixed deposit will take another few years (usually around 10 or so, but again depends what you are getting for your power). After that you actually start making money and the system will produce power from the sun for another 20 years or so.
So these are long term investments not short term and need to be seen in that light. Too many spend the capital and then forget that they did so and just look at the revenue rolling in and imagine that as 'profit' - and that's how the solar sales people sell it too. We do the same sort of thing with our car - we spend $15K, $20K or $30K or whatever and then pretty much focus on the cost of petrol as though that's all the impact is. In reality there are many other costs including the cost of the money and maintenance insurance etc - so the real cost is much higher per km than we think.
Solar PV is the same. I have 3kW on my roof and plan on putting up another 7kW, but I know why I am doing so and what my returns are and when I will get them and what the costs and risks are. You should judge all those things too.
In the ACT we have subsidies that make being an early adopter worth looking at - prices of these systems are coming down and energy prices up so in a couple of years or so (I reckon by 2015 or so) then the numbers will add up without any subsidy - so there is no real reason to rush unless you value highly being the amongst the first.
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.