well in Oz I understand there are no rules on tyre width for a rim, instead we rely on manufacturer deciding what is safe. so you will need to check for a given tyre to be sure. In general I would expect 245 is the widest you can run on an 8.5" rim
there is a full semi slick thread elsewhere, I mentioned those 2 because they are the cheap ones, between 300 and 400 a tyre. the best ones Dunlop DZ03G, Yokohama A050 or the Hankook whateveritis would be 500+ each.
But the grip will be chalk and cheese compared to road tyres