The flammable limits of propane in air are 2.4 - 9.5%, not quite the
50/50 mix mentioned below, but John's point is still valid. Unless
you have a leak, or are leaving your torch valve open for quite long
periods of time, it is very unlikely that you will reach the
flammable limits in the ambient air. There are a couple of important
things to consider, though.
The exhaust system you're talking about will remove air from the
room. It will be important to make sure that sufficient replacement
air comes into the work area, otherwise any exhaust will be
inefficient. An open window opposite the exhaust will often suffice.
McMaster Carr (http://www.mcmaster.com - search for part number
5159T41 ), sells a variety of hazardous location fans. Their
DC-powered fans are reasonably economical. Since they are 12 VDC, you
would also need an AC-DC converter and associated wiring. Still
cheaper than the available AC fans, I think.
One last thing that may be important to keep in mind. Propane is
heavier than air. Obviously, this means it will sink to the floor
where it will accumulate. Your exhaust system should have sufficient
airflow at the source of the propane emission to capture the gas and
vent it outside. If it doesn't, then the gas may accumulate low in
the room where most vents won't get at it. Now, my opinion (standard
disclaimers apply), is that you still won't build up enough gas to
cause any problem during any reasonable usage. However, I thought
that this point was important to consider, especially since the
workroom is in your basement.