It's only a hobby.... it's only a hobby....
There is likely a model rail club or more in your area. See if they have open house and go check out the place. A good club will be warm and welcoming with folks that are comfortable to be around. Forget the layout at the club, you are looking for a place to hang out with others that know a lot and don't mind answering questions. Fastest way to kill enjoyment of the hobby is to get around folks you'd rather paste in the beezer than talk to. Model railroaders are 25% normal people, 50% kind of quirky, and 25% jerks, given a random sample.
First step is to decide what you want to do. I like to do 1900's, 1960's, and modern railroading, so my layout is designed to be able to add and remove things that are not period. So the box car icing station from the 20'-60's becomes a fuel rack by removing structures and putting others in it's place. Also, do you want to be able to turn a train on and let it run around on a loop, or is it going to be a point to point layout?
Final advice - your relative may say maybe even before their death - but I wouldn't take over anything much before then. Just enough to comfort them they have gotten to bug to bite you is about right. If they give it all up too soon, they may be giving away what joy they can still get from the hobby. Just my two cents.
Digital control for more than a simple model is almost required. Most folks go with DigiTraxx system for controls, and various others for the decoders depending on what is desired out of the model. If you want sound, lights, and smoke (the good kind) out of a model, then almost certainly the decoder choice will be the appropriate SoundTraxx Tsunami - about $100 for the decoder, speaker, and supplies. Retro fitting decoders is fairly easy - the hardest part for me is getting the engine opened up without breaking off things.