I'm a little behind here and just catching up. You're going to get many different opinions. It's up to you whether you flush. As was said, if there's sediment in your coolant, then flush. I've never had that problem probably because most Ford owners know the penalty for failing to maintain their coolant. I would not do a chemical flush. Walmart distilled water is very cheap. If your coolant has rust particles or other sediment in it, fill up the radiator with distilled water and run it at operating temperature for an hour, then drain and refill, repeating the process 2x or 3x. The distilled water should look better with every flush. If it doesn't, it's not going to get any better and I'd cross that bridge when and if I came to it. Otherwise fill with 50/50 DEAC and distilled water. Test strips are available at NAPA. The last ones I bought were a Cummins Fleetguard product.
The Ford dealer won't have 134D. Valvoline is expensive but it will last you many years as long as it doesn't get condensate contamination. A suggestion was made previously that you use hydraulic fluid in the transmission and center housing. Do not do so. You need UTF (Universal Tractor Fluid) that meets or exceeds 134D in the transmission and at least a UTF that is recommended for use in applications requiring 134D in the center housing. Your machine has wet brakes and you need a friction modifier in your center housing fluid. Friction modifier may not be found in standard hydraulic fluid. I run 134D in both compartments because there may be a swapping of fluid between the transmission and center housing. There's no reason to risk your transmission for $100. Your don't know the amount of wear on your running gear and you don't need it wearing out prematurely.
You should remove your outer air filter and take a look at the inner filter. If the inside of the outer filter is dirty and/or the inner filter looks dirty, replace the inner filter. No use dusting your motor for the cost of an inner air filter. It's your second line of defense if the outer air filter is not maintained properly. A glass shop that supplies truck glass can make you a replacement panel. Auto glass is probably better than the OEM panel. We use it in all our CAT air cabs. A fraction of the price of OEM and a much better product. If you poke a hole in it, a piece of tape will allow you to keep going until you get time to replace the panel. $10,000 is a good price if she gives you 500 hours of service before something major happens. Clean air, clean oil and good coolant will get the most out of your investment and avoid premature failure.
The admonition that only the dealer can service your air filter is BS. You have to pay attention. If you don't install the air filters correctly, they will not seal and you'll dust your motor. Just take your time, use a flashlight or reading glasses if your eyes are like mine. Observe how the filter is installed and make sure it seals when you replace it. No big deal. You can run Travelers Premium (@ $40/5 gal) if your transmission and center housing is full of condensation (turns fluid milky white) but you'll need to drain and refill with 134D ten to forty hours later. If your fluid looks like new fluid, it's your call whether you change it or not. You won't be able to determine whether it's 134D equivalent or something less. I like safe rather than sorry. You won't have any fun rebuilding your shuttle or brakes. Now that you have an owners manual check the capacities section. I think the transmission and center housing will take the better part of 15 gallons if you drain the converter. You can run Travelers Premium in the hydraulic system.