ventilation is hard isn't it? esp. if you're not sure how much you want to commit to this artform yet and if you're trying to set it up on a budget... LOL
All i can offer is my own experiences, my studio setup was similar to yours:
http://shebamakeda.com/my-studio/ . i've used a double fan in the window for the last couple years, however, i wore a P100 respirator and didn't torch full time - more like once or twice a week for a few hours (if that) each time. I also avoided some of the more dangerous, i.e. fume heavy techniques like fuming, silver glasses, etc. If i torched full time i would have definitely invested in a proper ventilation system right away.
i wouldn't leave that window completey open, something is bound to either fly in, commit suicide in yr flame and scare the pants off you or i'm thinking a bird, squirrel or some other animal might decide to visit. potentially cute but pobably not the right time for furry/flying visitors since playing with fire is not the best time to be distracted.
At least attach a screen in the window so you don't have to worry about unwanted visitors.
2nd, you may find the light in front of the window too bright on sunny days. i'm set up in front of window as well and for a while couldn't work until the evenings or early mornings cuz i couldn't see the flame. so maybe find something to block out the sunlight when yr working while keeping a focused table top light.
3rd, do you have air coming from another source at least 10ft away from where you're exhausting? i torch in an upstairs bedroom so i keep the window in a bedroom down the hall open (yes, even in the winter) to ensure a regular fresh air supply
4th, invest in smoke & carbon monoxide detectors. you may also want to have propane gas detector if you ever decide to move up to bulk tank
5th, (and i'm being picky here) you move those extra propane tanks on the side far away from yr work area when you torch right?
i didn't see your previous setup but i know each of us has to make our own choices based on our committment and budget while still paying attention to safety as much as possible. You'll know if yr ventilation is inadequate by the feeling in yr chest , mouth or throat after a torch session - don't ignore the symptoms.
since i moved from a HH to a megaminor last month my current setup doesn't work anymore (cuz now i torch for much longer) so this past weekend i had a 650cfm fan wired and will be getting the tubing hopefully this weekend. it's a portable system made for temporary studios like ours. once i have it all figured out i'll post it here for others to benefit...