The SIR visa has very few restrictions when you really look into it.
Yes you can buy a house when you want, as Tyke says we did this in 3 days. You have to get FIRB approval but you just have that written into the contract(it can take upto 28 days but ours only took one week)
Yes you can buy a second house but it has to be Brand New.
Yes you can set up your own business all you need to do is apply for a ABN (australian business number)
and away you go.
Don't be put off by what an agent says. A lot of agents know very little about the SIR visa.
If you need any more specific info just ask