1. March-May is one of the two best times of the year, the other being October-December. The latter seems to have clearer weather though colder. I think you'll have relatively warm temps, especially lower down. Expect clouds later in the day and even some rain. The Everest expeditions will be in full swing, so its a good time to be out there.
2. Have a look at some of the links from other sites in the Nepal trekking section in the Downloads. If you have a Garmin and want the full works have a look at
www.nepalgpsmap.com . Before you make big purchases though bear in mind that in Thamel, Kathmandu, you find a wide range of affordable, good-quality trekking maps at every street corner. And the main trails are well established with high traffic, so its easy to get info from others on route.
3. I think look at about 1000-1500 Rupees per person per day for food and lodging (excluding guides, porters, travel, permits etc). I haven't trekked in the Everest region for about 6 years and I know inflation has hit Nepal hard. I've worked on daily budgets less than that previously. Currently don't have a very accurate feel for prices in that area. Remember too that prices will increase with altitude! What you pay in Lukla will not be what you pay at Namche and definitely not what you pay at Lobuche or Gorak Shep!
Am envious! Enjoy and let us know how it goes!