People don't find dinosaur fossils on Earth during weekend expeditions, finding fossils microorganisms on Mars will be tougher. That seems a good bet. We now believe that Mars was once warm and wet. If life evolved here, it might have done so there. But I think we'll find them, because conditions on Mars and earth were similar for a longer period of time than it took life to appear on earth, and nature follow lawful processes.
Nucleated cells are unlikely.
Macroscopic complex organisms even less so. We can't use the environment as a reason to go, because it's not the reason to go. We should go to Mars to explore, to find out about the place of life in the universe, and to begin the process of spreading life throughout the universe-our kind of life, that is. The American people will buy that, because they have already bought that. But every thing you learn about one environment will tell you about another.