Standard advice is to get a system that guys around you use so you can ask questions if you need to. Usually you have to ask a few when you get started.
Consider the amperage you might want and sound locos use more than just motors. Basic full size HO systems are usually 5 amps which should be fine for you. For O scale if want to someday get some bigger locos, there are 8 amp, or 10 amp systems. Digitrax O system is 8 amps, NCE is 10 amps.
You can add boosters too as you need them at least with a full blown system from NCE, Digitrax, or Lenz.
Something to consider as you do all these jobs is system with radio throttles. That way you have no plug in cables to worry about while jumping on and off the loco to throw switches or running into the tower to talk to central dispatching.
