I haven't been to Tunisia in March, but there are sites like Yahoo and wunderground.com (this site also shows "historical" weather, you can check March several years back) where you can get weather updates. Any hotel can help you arrange trips like that. You can also check
www.tunisievoyages.com - click the English button in the top right hand corner if you prefer English to French - where several packages are listed, with prices. I've been to the Sahara twice with them, but those were 2-day trips with a lot of other stops as well. So, I can't speak for the other trips - they have more than advertised here - just e-mail them, or contact your travel agent. Hope you have a lovely trip!