Welcome to the wonderful world of Thomas! I have an almost 3 year old son who loves him and all his friends. We have been using the motorized (battery operated) ones with the blue track. Primarily because my son loves to watch them go around the track and is fascinated with figuring out how they work. My only complaint about them (and this may have more to do with my son than the actual toy) is that he has figured out how to take of the lids to remove the batteries in some of the trains. Some trains require a screwdriver to get to the battery but some don't.
We have taken the trains and tracks on several trips to see family and they have always come in handy. I have been able to find new and like new accessories (bridges, tunnels, r/r crossings, etc) on ebay and craigslist. I have also found some of the engines for less than walmart price at big lots, marshalls, and ross. You just have to be willing to look.
One last note, I recently noticed that they have come out with a new line called Trackmaster, I think. The only difference seems to be the track, which is a plastic brown one, not blue. The engines will work on the blue tracks because I recently bought one and all we have are the blue tracks.
Good luck and have fun!