For heat and smell I would recommend a centrifugal fan and a canister carbon filter, there is really no substitute for it. They are rather expensive but try ebay. That alone will cost you but it's really an investment and for a cabinet that size, the only way to do it and keep it stealth. For an air intake I recommend darkroom louvers, you can get them online at a photo supply warehouse. They let air in but don't let light out.
Doran DV-99 8x8" Light Tight Darkroom Louver for D-68 and DVM-199 Fans
I would also recommend an air cooled light fixture, they aren't as expensive as they used to be and they keep the heat LOW. Plus you can get your plants a lot closer to the light, I recommend this model:
400W & 430W HPS Grow Light Systems > 400 HPS Cool Breeze Grow Light System
It gets you the most out of your light and cools almost as well as a cool tube. But that is if your using HPS (which I'm guessing you are). You don't HAVE to use an air cooled fixture but not having bought one for mine, I regret it a LOT. Still, you can save a good few dollars and get a batwing fixture or something similar.
Now keep in mind, to use one of those air cooled you have to link everything in line; fan, light then filter and the fewer bends in the hose the more airflow you will have, not always easy to mount that way, or mount at all! So you can save a lot of labor and go with an old-school fixture.
I would also recommend a galaxy digital ballast (or similar fan-less digital ballast) because you can actually mount them inside your cabinet and you don't have chords going everywhere and they can be mounted easily and are low profile. Plus that way you can use metal halide without an expensive conversion bulb. Helps keep things stealth and plant growth compact. Otherwise you may have to have your ballast outside of the cabinet and not mounted at all (the walls of a walmart cabinet can't take the weight of most coil ballasts) and thats more holes with wires coming out that you will have to plug up for flowering, and that is a bitch.
Now I know what I have recommended is a little expensive, hell more then a little, but if you want to be growing bud in a nice cabinet you don't have to maintenance for a good few years, spend now and pay less in the long run... but thats just how I swing
P.S. You could get MASSIVE yields from this cab if set up right, so you better post your future grow!