Agree with Gunny, regarding the large dealerships. Kubota isn't perfect, but it's why I eventually changed from Deere to Kubota. (In my area of the state, Kubota is more likely to be from 1-2 dealer operations, not chains/regional. The Deere dealer is now a 3 state operation, no other dealers for 120-200 miles in any direction, and couldn't care less about you, unless you're buying something at a $200k or up price point…).
Regarding the original question, I have an 8' Modern Ag that is fully galvanized. I believe Land Pride is making some galvanized shredders, as well. This thing is 9 years old, and still looks new. Real shame that more manufacturers don't do galvanized, instead of paint. It's also got 7 ga sides. Basically a heavy-duty model, at medium duty price.
There's obviously more to consider than just a galvanized deck, but I can tell you that it's definitely a great "feature".
No matter what you do, invest an extra $150-$200 in a good leaf blower, and get religious about cleaning off the deck after you mow. At $15-$20k plus, for a good batwing, it's worth the extra 5 minutes to take care of it..