A good transformer to start with is the 1033.
Identification details about the Lionel Trains Type 1033 Transformer that Lionel made during the Post-war period.
It will easily handle one train.
If you are looking in the future to possibly run more than one train on multiple loops, then a KW would be recommended.
Identification details about the Lionel Trains Type KW Transformer that Lionel made during the Post-war period.
For more than 2 trains, or if you will get accessories, than the ZW is king.
Identification details about the Lionel Trains Type ZW Transformer that Lionel made during the Post-war period.
All have a whistle controller, with the ZW having two.
Larry