Well, first develop a plan for the layout. with an estimate of size, and items you may need. The plan should include a track plan, and a schedule for its construction. Rome was not built in a day, start small and show how it will grow over time. Present it like its a business proposal, with the end goal in mind.
Include solutions to any concerns (like your parents may need that space sometime, or we may move some time). Explain why running trains on a carpet is not the same as a detailed layout.
Some of the material you may find cheaply. For example, old, used doors, sometimes can be found at stores that sell custom doors. Hollow-core doors are light weight, two doors would make a great start for a layout. Design the layout so it can be expanded, but also portable so it could be taken down quickly.
For benchwork, old kitchen or bath cabinets can be used. Ask around to see if anyone you know is going to have some remodeling done. Old bookcases can also be used.
Be creative. For example, if you make a mountain, make it such that you can remove it easily, I did that using foam core as a base.
Keep us posted and good luck.
