Very true, I don't know why anybody uses them unless they just don't know any better. Or at least, I can't think of a reason myself. With the exhaust air blowing out it automatically sucks air in meaning a fan blowing air in wouldn't make any difference at all except like you mentioned with extra noise, energy etc. I currently have a similar setup which goes phresh filter -> light -> fan -> blowing out. But whenever I ran a bigger room with more lights I always ran a separate fan for the lights that went through all of them, and a separate fan from the filter going out. To do this without having an extra filter for the lights you need to start with air that isn't in the grow room, so that no air that goes through the lights can come from the grow room and smell it up. I am not sure how much a filter slows it down but now that I am only running one light it works great.
Also, those VenTech fans I mentioned work very well for me. I have like 4 of them I have bought over the years and one of them is a little noisy now but still goes strong if I needed it. But since I only need one fan right now, I use one of the best ones. Good thing about them is the price is right for around $60-70ish to your door. I would love to have one of the more expensive fans that is suppose to be quieter though, just not sure it is worth it. I have insulated mine some and the sound has gotten much quieter but I still plan to build a box around it and enclose it with insulation. Also remember a 6" fan that is turned down in speed is quieter then a 4" running full blast. If I had to do it all over again I would have bought 8-10" fans instead of 6" ones and just turned the speed down on them so they still pushed as much as a 6" but much quieter.
And lastly, do NOT skimp out on your filter system. I know many do and buy some cheap off brand filter or try to make their own.. but trust me if you live in an area where smell matters just pay the price to get a Phresh filter... they last about 2 years and are awesome at getting rid of even the smelliest plants aroma. People also love the Can-filters but they are about the same price and much heavier so I always went with Phresh. For as small of an area as you have you could probably get by with a 4" fan and filter, but I highly recommend going 6" if you can swing it and just turn it down some that way when/if you ever go bigger you will have room to upgrade.
