Actually, with the cheap cost of really good furnace filters, a good fan, duct tape and drop plastic it's pretty easy to create a positive pressure in a room; assuming you want to go all MacGyver and all :-)
As far as masks go, any surgical grade mask will work in the short term. The radioactivity isn't necessarily floating around in the air. After the initial thermal (big fire ball) and Gamma burst (which nothing short of lead underwear or three feet of dirt will stop) it's the residual radioactive particles of dust and soot that pose the most biggest danger.
Hmmmm...I never saw a furnace filter that didn't pass some particles. They will filter fibrous materials such as lint, but not fine particles such as soot. Normally, that would not matter, but this time we are talking about radioactivity passing through. I suppose inhaling even a single "hot" particle could initiate a cancerous cell mutation in the lungs. Then there is a matter of the fan. It takes a strong motor to pull a volume of air through a fine filter. We're not talking batteries here, but A/C current. After a nuclear blast, what are the odds the power plant, lines, substations, etc. will still be intact?