I keep a bit of everything on hand. That said .020, .030, and .040 work for most walls, etc. Heavier pieces for the base.
I keep a large supply of strips, rods, tubes and shapes in a box with vertical dividers for easy finding. Go visit the Evergreen rack and buy what looks useful. Be assured the one size you need at night is the one you won't have (after the hobby shop closes).
If you use the score and snap method for walls laminate them to .010 as you reassemble, then cut the openings through the .010. This helps to strengthen the whole wall.
Another tip is to drill a hole in a rubber stopper, inset a fine brush and use this as an applicator for the liquid cement. The stopper should fit the bottle and serve as a cap while working on the model. Put the screw cap on at night. The brush in the cap is usually to big for fine work.
Don't forget to shave the burr off of a score using a chisel blade. Do this EVERY time you make a score so that it become automatic.
Styrene is great to work with. Good Luck and have fun.
George Lasley