not an expert, but I would have the both vents on at the same time.

Reason for this is simply because your lights are going to be on either 18/6 or 24 hours a day. If there is nowhere for the heat to go, it will get stifling... The light's wont produce an intense amount of heat, but they could easily burn your plants.... Plus if its in the loft, then the insulation could ignite.
It's not going to cost a LARGE amount to run them.. Dunno if its enough for 40 plants, guessing you would do SOG, in which case it should be alright I suppose.

To be honest, sounds like your planning on dealing. Nobody goes from nothing to growing 40 plants. Plus more plants = more chances of males = more chances of fertilization, not to mention possible bugs, illnesses and whatnot, so you need to KNOW what your doing, growing for yourself is one thing, but wanting to supply to someone else means your gonna have to know what your doing.