Here are a few comments about what you said. FWIW, I’m a programmer by trade, so I understand where you come from 
- You really shouldn’t try to use custom, hand-made collision boxes unless you have to. Like you said, it’s a time sink. Every asset you import without collisions will get an automatic one in UE4, and you can further optimize them in the mesh editor.
- Textures are not typically shared between assets in “modern” games, rather, they tend to have a unique set of textures (normal map, albedo, etc). But that’s not the most pressing issue and that really depends on what you’re doing.
- Memory will be fine with 50 assets instead of 1. Actually, since 20 of your 50 assets will be instances of the same wall section (for example) you will save a lot of memory space for the same quality. Don’t optimize before measuring.
- Snapping works if you designed your assets to use it. For example, I always use power of 2 dimensions : a 5 meter wall becomes a 512cm wall, I design a 512 unit asset and that’s it.
- Instead of 3DS or Maya, you can use Blender, which is free, and really not bad for game development. It works pretty well with Unreal - save, hit file/export/fbx and voilà . All my game’s assets were designed with Blender (take a look at the workflow here).
The one thing you could do is show your project here. Show us what’s wrong, what your assets look like, how you import them, etc. We can help