I am an installer. a few things to note.
-i worked for sunelec in clontarf for a year (iv moved up north now) very reputable. highly recomended.
-your house uses power before you sell it, on todays tarriff you get 8c per kwh (kilowatt hour) sold (16 with AGL i believe) so the power is more valuable to you used (meaning you dont have to buy that kwh for 25c), so try to use more pwr during the day, washing, pool pump, etc.
-reputable inverters are generally more pricey, do not buy cheap. good one's that i have personally installed and never had a call back to are SMA and Aurora. and decent panels i know of are bosch and conergy, but everyone will have their own preferred panel to install.
-as someone else said look at the inverter at least once a week, so you'll notice any red lights.
-the physical limitaions of the building WILL affect the performance, but the installer should let you know all that before hand.
-4-5kw will should get rid of your power bill (i assume its around 300ish? for 2 peeps) plus a bit sold. they give you a cheque once a year, to pay out what you sell to them.
-thats about it man, go for it power price will only go one way, ^, but the amount you produce should sty the same for at least 10-15 years, and the amount you use should stay the same if not go down, as more efficient appliances are invented...unless you have kids of course.
feel free to pm if you want more info.