Well, if you've never tried the MOPP gear, or hazmat suits, you might be surprised how fast you'll sweat out by trying to have a full get-up. I don't know how well PVC suits and duct tape would work for keeping you clean, but I can assume that, after a few hours, you'd be dead from heat stroke, unless it's the dead of winter. If, or course, you didn't die in the first few minutes while you're fiddling around with duct tape sticking to your rubber gloves.

As for gas masks.... I'm no expert there. But, if you go and buy one, better get LOTS of filters, since they only last a set amount of time. You better learn how to change that filter without exposing yourself to whatever you're protecting against. You also better buy some canteens with the nozzle adapter so you avoid the heat stroke mentioned above.

YMMV, but it doesn't seem practical to me. Just as an aside, I have seen the charcoal lined suits at various surplus stores/websites, if you're really hell-bent on buying something for an NBC environment.