Depends on how big the things are. If your house is doll sized, the whole house can be a mesh. If you’re just going to be on the outside of the house, ditto. However if you want to go inside (and make it big enough to be able to go inside), it’s better to split it up so the engine can cull the parts not in camera view.
Everything depends on what your goal is. For instance, I know my computer is not very good, so I focus on AI/level generation code and not meshes/textures/whatever. I set up the level using default meshes within UE4 and once I was satisfied with the general layout, I imported meshes and textures and replaced the Set Static Mesh template cube with Set Static Mesh skyscraper.
Failing to make backups/use version control.
C++ is more performant. However if you don’t plan to have 1000s of AI at the same time in the level, it shouldn’t matter.
As far as HDD space goes, UE4 itself wants quite a lot. Then there are the backups it makes - those can easily go into multiple GBs even for a fairly small project (200+ MB). While they might be useful, I am using a version control service so I prune the /Intermediate and /Saved folders from time to time. As far as RAM is concerned, watch out for physics and culling. Especially physics. But in general, don’t worry about it unless it’s an issue.