1. Water in the desert during winter requires the same attention as in spring. Assuming it rains in the desert, which is a rare event in winter too, the water in pits and temporary ponds, is not potable and requires purification.
2. Sleeping bag rated 30 is not sufficient. If you bring a tent you'll need at least 20 for the entire trail. Temps in the desert may get as low as 32-35 and with a little wind it's cold.
3. Cost of a caching trip in the desert: $500 (NIS 1900-2000) looks to me a bit expensive. If you talk to other people I believe you can get down to $375-400, which is still not cheap. You have to realize two things: It's not a ride of 250 miles on I-90, it's a ride on dirt roads, accessible to 4X4 only, and it takes a full and very long day. For a single person it is expensive. Usually you share this ride with other people, and the price per person is not more than $100, usually less. Problem is that in winter fewer hikers hit the trail so your chances to share the ride are lower. All drivers in the desert try to make the caching trip for several people, unless you insist to drive alone.
4. After answering your questions I'd say that "normal" preparation should not intimidate you, thousands of hikers do it each year. Winter is colder and naturally rain delays are more frequent, but there are hikers in their right mind that do it in winter.
5. Here is an e-mail of a 23 years old man, who starts on December 13th: gwiths@gmail.com
Enjoy your hike on the INT, whenever you decide to do it.