re: How long before Mars Express looks for water/ice at Gusev Crater?
From memory and I've seen this stated in several places:
The Mars Exploration Rovers (MERs) will not actually look for present-day water. Rather they will search for the effects that liquid water in Mars's past may have had on rocks and soil.
From: http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=228
So, it's strictly a geology mission (with very good photography equip.).