I did with this company the Everest Base Camp Trek, we were two people and we can affirm this was an incredible experience. You can plan all the trek with the company by mail, choosing stages and stops on the trail.
We had a fantastic guide (Pasang Dorge Sherpa) and porter (Ang Kami) who helped us during all the trek. The guide start with you from kathmandu and you have to pay his flight but the porter started from lukla.
I thing you will lost 50% about your experience without guide and porter and without entering in contact with sherpa people.
The agency is located on the second floor of a building under reconstruction, it's not so easy to find!