The reason the canister has to be purged is that after a while it will fill with vapors and will smell of gas. It's not a big deal to install one, you just buy a canister and a canister control valve and hook it all up. I'm having a difficult time picturing the rest of your set up and how it would work properly. There is a filter on the bottom of the charcoal canisters I'm familiar with. I'm not sure what you mean by running to it's own air filter. What happens is that when the purge valve opens fresh air is drawn into the canister through the bottom through the filter. That air mixes with the fuel vapors as they go into the engine. A check valve in the vapor vent line prevents pulling a vacuum in the fuel tank. The only problem I see is that the system I'm describing is an older GM one and you may not be able to get parts for it anymore. The newer ones are controlled by electric solinoids activated by the ECM. In any case I'm no instructor and sometimes don't explain things very well. I think the first thing you have to do is have a complete understanding about how the systems work then I'm sure you can design one easily enough.