Choice of Plastic, Glue &

What to do about thickness?

I've been comparing  Styrene with PVC (type 1) and Expanded PVC as building materials but don't know enough to make a choice.
Expanded PVC is sold as Sintra, Celtex and Komatex (brand names).  In checking with a few suppliers, Expanded PVC is typically cheaper than Styrene.

Here are the material specifications I've been able to find

sp-2010 Celtex typical

hips expanded pvc type 1-pvc
density gm/cm3 1.04 .55-.70 1.4
Tensile-strength psi 3500 2253-3000 7400
Tensile-modulus kpsi 239 144-232
Flex-strength psi 6900 3329 12000
Flex-modulus kpsi 277 144 400

Based on this, if I'm reading the numbers correctly:
It's been pointed out that strength is an issue with Dave Everett's Styrene droid plans. I would not consider substituting expanded pvc for styrene for the critical parts. For non critical parts, like the skirt or the shoulder horseshoes, its worthwhile. From experience, Expanded PVC cuts cleaner than styrene.

There are a number of cements that IPS recommends for styrene/pvc. From experience, either Weldon #3 or Weldon #16 produce excellent bonds between styrene-styrene, pvc-pvc and pvc-styrene. Weldon #3 is clear, water thin, very fast curing solvent cement. Weldon #16 is clear syrupy (medium bodied) cement for high strength joints.

The Styrene Droid plans published by Dave Everett call for sheet Styrene in metric sizes, 1mm, 2mm and 3mm. Sometimes however your local plastics supplier only caries 'English' dimension material .040, .125 and .250 inches. As many folks, on various web sites, have pointed out the difference in dimensions is small

metric English matches difference
1mm .0394 .040 .0006
2mm .0787 .080 .0013
3mm .1181 .125 .0069
6mm .2362 .250 .0138

If you are substituting .125 for 3mm then the difference is only 7 thousand's of an inch. If you are cutting by hand that difference will be lost in the accuracy of your cuts. However, I'm going to be using a 3d cad package to design my parts. Since small difference play havoc with mating parts the computer parts are all designed assuming English measurements (.040, .125 and .250 thicknesses.) Layouts will be done using the part centerline so that dimensions remain true to R2BC drawings.