Wow.....lots of stuff to say.....there's a lot of info out there and yes, it can be confusing.....
1st of all, welcome to the OGR Forum.....a great place to be if you are in the O gauge hobby....
In no partcular order:
1. Lionel has been around since 1900.
Prewar refers to trains made before WWII or 1942.
Postwar trains are those made from 1946-1969.
1970-to the present day is called the modern era.
2. Yes, you will need a 2nd transformer if you get a second "conventionally controlled train set". That is a train that is dependent on the votalge lever of the transformer to control it.
Just to make it more confusing:
There are command control engines in the Lionel arsenal.........one group called LionChief/LionChief Plus and the other called (different mechanism) TMCC/Legacy. Bothrun through a special handheld.
Price is what you are comfortable with......I would go to a local shop, go to local shows......get and read the 2 major mags OGR and CTT....
Go to the lionel website.......www.lionel.com. Look at the other main manufacturers.....MTH and Williams by Bachman.....There's a lot out there and just read up and have fun!
Peter