I believe the Accuspray system has a separate air intake through the filter and does not draw air across the motor which is good.
You also need to make sure that your hose to pump connection does not get overly hot as it could cause the hose to offgas potentially toxic vapors.
You are right in placing a y-valve to vent surplus air. 8psi is way too much. Many positive pressure SCBA systems have a static pressure of only .9-1.4psi in the facepiece. Of course the hood should allow extra air to exit without building up pressure.
I built a fresh air system using an Ametek motor and pump (the same as the one in the Hobbyair system), light ribbed medical ventilator hose and a MSA mask. It works absolutely great. With a full hooded bunny suit, gloves and the fresh air system I feel very safe from iso's (but hot!).
If you can restore a car, you can build a fresh air system. It's not rocket science but you have to use due diligence, understand what you are doing and know the risks you are taking. Actually, for most people it is better to buy a system as the cost of the parts can approach that of a ready made system. You have the motor assembly already so it makes financial sense to make your own. I lucked out on a great deal on the motor and I had the mask already.