plethora to me feels almost more poetic or figurative. Surplus feels more precise or quantitative. A factory might have a surplus of 5,000 hats, or a budget surplus of $2 million. But if you walk into someone's closet and see they have a large collection of hats, you could describe that as a plethora of hats!
If I read the following 2 sentences:
1. "The library has a surplus of Dan Brown novels"
2. "The library has a plethora of Dan Brown novels"
I would understand them as
1. The library has too many Dan Brown books, they need to get rid of some. A somewhat negative thing.
2. The library has a wide variety of Dan Brown books. A good thing, if you like Dan Brown books.
I only suggest this as a native speaker, I'm no expert in the language!