1. New lights. You can reuse incandescent or LED lights. May need to match their voltages to the decoder voltage outputs with resistors. Best practice is to do all LEDs.
2. New speaker. May be able to reuse existing speaker depending on its resistance value. Best practice is to match the speaker resistance with the decoder output resistance. Most often 8 ohms.
3. DCC decoder install. Yes if you wish to run DCC. No if you wish to run straight DC.
4. What will I have to do to get power from the wheels instead of the pick up roller? Common practice is to isolate the motor(s) from the frame and wire them directly to the decoder motor output.
The new 2 rail wheel sets will have one side insulated. Common practice is to install so the wheels in one truck pickup from the left track and the wheels from the other truck pickup from the right track. Then install wipers which ride on the insulated wheels sets to conduct the track power from one rail to one terminal on one decoder track power input terminal and the same for other rail to the other decoder track power input terminal. (For DC directly to the motors)
If you completely insulate the trucks from each other and the engine frame you can directly connect the motor wires to one side of each side frame to complete the track power inputs circuit to the decoder (DCC) or the motor(s) (DC). .
(Let's see. What have I forgot or gotten wrong? Someone will help!)
Oh yes, if you are able to purchase drop in wheel sets with the correct gears that's one thing. If you are going to use the same axles and gears and replace the wheel sets you'll need the tools or a buddy's tools to do so. And sometimes pressing the 3 rail wheels off can be hard to do.
Go for it. You'll learn a lot and it ain't rocket science. Just take it one step at a time and learn as you go.