I highly recommend that you get in touch with a professional as it sounds like you are not well versed in the mechanics of this type of project. While the guide you posted is a good starting point, there are many factors that need to be considered and their approach is not necessarily the best for all situations. You will need more than just a motor, you will need gear reducers and some kind of control system with a safety system. Usually this type of project is not just plug and play. If you look back at this thread you will see that the OP has someone with an engineering background working with them.
As you are in England I can't recommend any local people to contact, but I am sure that your local theatre supplier can point you towards the right people.
