Hi mate, converting a DM1 is hard. But here are some things that may help you. If you convert to single speed you will have to re-drill the center part of the chassis as the two speed is longer and if you are going to aim for 100mph is probably a better option especially with mechanical brake.
Before you go rushing into what packs and motor and speedy do some research and see how much room you actually have on the chassis. Are you going to run the motor on the right where the rx box is or are you going to put it at the left front so you can run two packs at the back of the car. the only reason i say that is the DM1 chassis is very narrow and does not allow itself well to have one large pack on the one side.
This is only what i found when i converted mine here is a pic.