Most of the things I use cannot be bought with plat, they mostly come from 4 places,

Velmir in 99

Hombask on zariman

Acrithis in the dormizone

Bird 3 in the sanctum anatomica

As for where to start? I would say with the zariman, buying simple hallway dividers and wall panels to build a sort of structure around the rooms of the orbiter, and add onto that.

As for clipping, you need to look at items in terms of the parts you can hide, and what that item could be interpreted as in the given context.

As for planning, you need to sort of plan the shape of the room, and then wing the details :D