You can get by with one, but eventually you'll enjoy the convince & efficiency two fans offer. I use the Vortex VTX power series now, but I started with Ventecs & those fans still work. In fact, I'm using one right now to dry the floor where the frigging dishwasher flooded.
As for the Ventec filters, they are trash imo. I know they're popular because of price, but the one I had couldn't filter an angel fart.
It didn't do jack shit. Nothing. But, maybe I just a got dud. I don't take chances on odor control, it's one of the holy trinity (no smell, tell, sell) for staying safe. Phresh works for me. Get bigger fan than you think you need and slow them down with good speed controllers.
I ended up buying cheap the first time, and that's ok if you have NO choice, but you'll eventually end up buying quality equipment.
It's the same with any hobby. Hang on to the cheapies for emergency backups.
I started with a 4' t-12 shop light, miracle grow & a couple of bag seeds in a corner of the basement. Now I've got 4 inlines, two hoods, two ballasts, two filters, multiple mh/hps bulbs, timers, half a dozen clip on fans, etc, etc.
It's not all bad, though. I still use those original t-12's for the first week or two of veg.