As the resident Finance guy at a Ford dealership.... Order what you want. A reputable dealer will indeed agree to a price for you upfront, and print it for you. You are always eligible for any rebates at the time of delivery. Most dealers will ask for a small deposit, typicallyl $500 or so. I'd sell the trade yourself, a 20 year old Benz will honestly be a wholesale unit, and you'll be looking at $2000-2500 if you are lucky. Not bashing the trade, I've been looking at trade appraisals for 15 years, and tend to know what the used car manager looks at.
Good luck! If you'd like to order it and get it in Vegas, let me know.