QUOTE:
is it better to stay with Joomla 1.0.x/Ario or try Joomla 1.5.x/Ario/Legacy mode ? or maybe the best is Joomla 1.5.x/Ario without legacy mode
You will get a lot of different opinions on that matter. I myself would say go with 1.5 because there is always a workaround or better component/module being made, or made for it to fill your needs.
Chris and Andrew will probably say that 1.0 is the way to go due to the years of support and excellent proven security that 1.0 provides.
Neverthless with any big site you build some decisions cannot be reversed and sometimes you will have to change plans midstream. I have myself built entire sites and had to suck it up an rebuild it with 1.0 for some reason or another. Try them both on a test server locally and see if all the components and modules you require install and work well, then make your decision from there.