I started buying mini's way back in the 70s, when they first became available. I generally prefer metal to plastic. Cardboard was never a contender.
For scenery I use a number of sources: The aquarium section of the pet store has things like roman/greek columns, bridges etc. There's a vendor I see at Cons that sells resin walls, intact and ruined. I've tried the Wizard's cut-and-fold buildings, and they look good, but scenery that stands that tall becomes a problem: Players can't see the figures over/around it.
I have a few bags of colored moss/lichen for brush.
I have enough figures to field several armies worth of unique figs, but I find that I seldom come close to using all that many. I simply have to acknowledge that I can't possibly have the right figure(s) at my fingertips all the time, so for play I generally have a single box of "people" types, for PCs and NPCs, and two for monsters. I often use the same small figures for Goblins, Kobolds and most other small humanoid types, another six or eight figs for Orcs and medium humanoids, a similar number for Ogres, Trolls and such, and a few for Giants. Half a dozen demon-types, a pack of wolves.
The point of all of this is that we're trying to help the players see the same image that the DM has in his or her mind, to enable us to quickly and easily draw/lay-out the places the PCs are in. Wall sections go down faster than I can draw lines on a battlemat, can be quickly shifted as the action moves on.
Doors are good (WizKids makes some that can be opened and closed), furniture less so: I have enough to do a minimal bar scene, and very little more.
My solutions may not work for you, of course.