How to build the ideal level editing mesh kit?

I am building a modular mesh kit for assembling levels inside the editor but I was wondering how many pieces I would need. Note that we are talking a generic general purpose kit with no actual textures/materials applied that can later be modified for specific uses. Are there any guides out there for building modular level design mesh kits for Unreal or even just modern game engines in general?

I kinda expect a full set to have ~80 assets, something like ~30 parts for walls, buildings, and other large assets. ~30 major props, chairs, benches, tables, light fixtures, etc. And about 20 things for set dressing, plants, cups, light switch, trash, etc.

How modular should the walls be? Currently I am only making individual flat parts of various proportions that can be lined up with each other for variety, but I also intend to add more complex stuff like borders, corners etc. Should I just make a bunch of pre-made borders for example or should I make individual border parts that can be used to make custom borders? How far to take modularity?