Just to first clarify, you said the dealer hooked you up with an a/c controller - I assume you mean an a/c transformer that will let you control the k-line engine. The reason I ask is that some/many LionChief engines come with a wireless remote controller that only controls the operation of the engine, but you will still need to provide power to the track, such as with your a/c transformer.
I agree that a Lionel Lionchief steam engine and tender would be a good choice for a starter train and, since you said you already have a lot of cars, you can buy just a "stand-alone" Lionchief engine and tender, such as the 2-4-2 series, and hook your existing cars to it.
In addition, some Lionchief engines not only come with a wireless handheld remote to control the engine, but also can be operated via Bluetooth on a cell phone or tablet just by downloading the free Lionel app; which may peak interest in young children.
As to the sound issue, putting a piece of carpeting, mat, indoor/outdoor carpet or similar deadening material under the layout should help reduce the noise level. Just make sure all your wiring is insulated and no bare wires or bare metal is touching the material.
