We did exactly this a few months ago. We gave them outline itinerary and they quoted all inclusive price. We were particularly pleased that our porters were not overloaded at any stage of the trek. This was definitely not the case with some other parties we saw. I have twice enjoyed a memorable trekking holiday with Roama Travel of Shroton, Blandford, Dorset.
Camping is the only way to see Nepal.
Avoid the more popular tea house treks that stick to well worn trails. They have no fixed itineraries, but will arrange anything you like. Intimate knowledge of Nepal and all profits go to support charities and schools in Nepal. It's run by a delightful retired Lieutenant General, Jimmy Singh, and is very efficient and also friendly, ecologically sound and very good to all the staff they employ. Also all your money goes to help local people.