In reply to bb1304:
I used to work in this area. It is possible to book a large group with no names and confirm details nearer the time. The cheap carriers (Easyjet, Ryanair etc) won't let you do this but you'll not be using them to get to Nepal anyway.
You will need to use an agent who will negotiate with airlines to get the best deal - when you're looking for more than c10 seats, the information on airlines' websites ceases to be useful re price. Usually, you'd agree on a contract which gives you a little wiggle room on the final number (ie you can lose perhaps 10% of the total seats initially booked, without charge). You will probably have to pay a deposit up front, with the balance due perhaps 4-6 weeks before departure. You may be able to negotiate excess baggage to be included in your deal.
I can recommend Jon, the Groups Manager at
http://www.colpitts.co.uk/services/ - we worked together on several projects in my previous job, which included a lot of large group foreign travel. The charges for agent services are not great (in fact you'll barely notice them as the group ticket price for a large group will probably be pretty competitive). The advantages of using an agent extend far beyond just getting the seats booked - they'll help with all sorts of problems, particularly if bad weather/ strikes etc affect any part of your air travel.
Hope this helps.