I wonder if you could have some form of government funded entirely by voluntary donations (and actually have something distinct from New-Mars-esque privatized judiciary etc)? You'd need the right sort of culture (clearly contemporary Britain is nowhere near); perhaps it could grow out of a society with a strong tradition of giving.
I don't think you should aim for a level of taxation; that's like aiming for a particular rate of car accidents, :). It's in balance against what it achieves (be that services or redistribution). Is it an income tax, or does it include sales taxes like VAT?