Hi Christiane
To answer your post – S&F is not currently designed the way you want – so really what you are asking is for some custom development.
For many users an “all items” option is the best thing because it is not possible to “deselect” a radio – in other cases it is not.
However, you CAN hide this option using CSS – there are classes for every field generated so you just need some CSS knowledge and.. http://www.w3schools.com/cssref/sel_firstchild.asp
To automatically select one of the options you have 2 choices – use Javascript to set the state of the radio on page load, or, instead of linking to your search page ie:
yoursite.com/property-search/
You could link to the results page:
yoursite.com/property-search/?rent_option=rent
(this is not the real url of course but an example)
You can find the real URL by enabling “bookmarkable URLs” and performing the search.
Thanks