Welcome to the forum!
You can never have too much power available for your trains and accessories! For an older transformer, the postwar ZW at 275 watts input power cannot be beat. With 4 independent throttles, you can run your trains on 1 throttle, and have 3 independently adjustable throttles available to operate your accessories with plenty of power to spare. No need for a separate transformer for accessories if you are running just 1 train.

The postwar KW transformer at 190 watts input power is an alternative with 2 independent throttles.
Good, used transformers are available at most Lionel dealers, and also online. Make sure they are tested and repaired/refurbished before purchasing one.

For a layout your size, make sure the track is clean and free of rust and dirt, including the connecting pins. Hardware store naphtha is the best cleaner/degreaser to use on our trains and track, and a Scotchbrite pad can be used to remove light rust. Do NOT use sandpaper to remove rust, as it will remove the tin plating off the track.

For better power distribution, a lockon every 3-4 feet of track is advisable, hooked up to a track power bus under the layout.

Larry