A P100 filter won't stop organic vapors and a positive air supply system is a pretty extravagant expense for this project! The "P" in P100 means it's a particulate filter that's effective against oil-based aerosols, but it won't do anything against vapors. You want a respirator with cartridges rated for organic vapors.
Search for "3M half-face respirator" and you'll see lots of reliable options starting at under $20. Here's the 3M respirator catalog. I've been very happy with my 7502 respirator.
For filter cartridges, here's a 3M brochure that explains all the options. What it boils down to is that you want one of the following types:
6001 - Organic Vapor
6003 - Organic Vapor / Acid Gases
6005 - Formaldehyde / Organic Vapor
You can get a 3-pair pack of 6001 cartridges for <$35. You'll know when to replace them; in my experience they quit filtering pretty suddenly so if you smell the chemicals, it's time.
If you want to filter particulates as well, get a pair of 501 Filter Retainers and a box of 5N11 (for non oil-based particles; analogous to N95) or 5P71 (for oil-based particles; analogous to P95) filters. The retainers hold the filter pads over the outside of each vapor cartridge.
Remember that the Poly Fiber system is Methyl Ethyl Keytone (MEK) based, so you absolutely MUST protect your eyes any time you open a container of Poly chemical. One drop splashed in the wrong direction can be disastrous, as MEK will rapidly destroy eye tissue. Consider a pair of lab goggles, or upgrade to a full-face respirator if you'll be spraying.
Finally, watch out for fakes. Buy your safety equipment from reliable sources.