When I bought my Woods 7' mower deck, I took it home, cleaned the paint/light dust off, and coated the entire deck with a farm supply product like Lubriplate. Now - 4 years later, you can't tell I did anything to it. My point is that I'm not sure much of anything will keep rust at bay under a mower deck. For all the abrasion they get, coupled with the moisture, I think rust is a certainty. The only mowers I've seen that have rusted through have been 30+ years old, and sat unused for extended periods.
For $200, you'll get so much use out of that mower, regardless of under-deck treatment, that you're far ahead of the game just to clean out the deck everyonce in a while. You could, as someone suggested, get a bed-liner sprayed in, but at that point you've sunk some serious money into what was probably supposed to be an inexpensive purchase.