For starters, a cheap way to make a decent heater would be nichrome wire, or just buy some cheap heating unit that you can control from the control board or has its own temperature conroller.

From what I understand, HEPA filters are physical particle filters, and don't help with toxins specifically. Activated charcoal is a chemical filter and can actually neutralize toxins. I think the best solution would just be a vent to the outside or good ventilation. I can't say whether either HEPA or charcoal filters are appropriate for what comes out of a 3D printer.