I guess we all have our preferences on what system we want to use and what we think works best for us. I use the RailPro system which you only need a the Hand Controller HC-2, note here you can just use the computer interface module which is around $35.00 and save the price of the HC-2 at around $300, but this does require using the computer to run your engines. A LM-3S-G module, a charging wiring harness, Lith/ion battery in the 3.5 14.4 amp range, a speaker. I personally take out all the factory electronics and pitch them aside, including all the lighting if it is not LED's, just not worth dealing with old style lights. I also like the AirWire System, but the cost of RailPro per engine is less and much more compact to deal with. I use RCS of New England for all my RailPro needs, Don Sweet will work with you and his prices are good. RCS wiring harness will give you one charging port for the battery charger and one switch that you can switch from run to charge, which is about as simple as you can get. RCS will also have a module stand offs for the LM-3S-G module, this will suspend the module where you can run wiring under it and will also help cooling the heat sink on the module board. After you have purchased your hand controller, the cost per engine is going to be around $265 to $280 depending on battery size, etc. to equip each engine.
You might like the YouTube video on the RailPro install, https://www.youtube.com/watch?v=zFyuK7r7w-k
trainman