General approach for level design.(level streaming, lods etc)

Hi all, I have few questions about general project setup.

I’m working on a small game/demo that takes place mostly in one apartment, staircase(4 floors but all looks the same and only this one apartment is enterable) and basement(straight corridor with few small branches).

First of all, how to approach level building? Is it worth using level streaming for not so big environment? I thought about making every floor separate level (for now I have them as separate meshes), and maybe even every room. Or better go for separate walls?

Second thing is I want to change this later on, like add new textures, meshes etc(something like another dimension/time travel when everything is similar but different). Again, what is the best way to do this? Replace it on the go, or build entirely new level and teleport the player there?

Another question, do I have to do lods? Personally I don’t think it’s necessary in such small and closed enviro, but who knows.

Thought it’s rather simple project, I’m a newbie and I’m not sure what is best.

I’d really appreciate some help. Thanks.