Transformers do not need to match in wattage.

You will need a minimum of 3 transformers - one for the main loop and one each for the ascent and descent grades. You may need feeder wires on the main loop to even out the voltage on the far side.

Used transformers from a reputable source are a good deal, especially the big postwar units like the KW (2 separate throttles for 2 trains) and the ZW (4 separate throttles for 4 trains).
http://www.tranz4mr.com/O_For_Sale.html

Here is a tutorial in phasing transformers.
https://lionelllc.wordpress.com/projects-and-tips/wiring-your-layout/phasing-transformers/

Hope this helps.
Enjoy your trains!


Larry