Nothing but love for my 02 Excursion 7.3. Has 230,000k on the clock and running like a top. I also have many mods not because it needed them, because the 7.3 addiction is real. lol Anyway I have 35" tires and 4:10 gears, seeing anywhere between 13 - 17 mpg pending driving conditions. Winter diesel you will loose MPG. Best I have seen in stock configuration was 27 on a 2500 mile trip. Its recorded in IOS Fuely App. so not making it up haters. Lift, tires, loaded down and now a roof rack and mpg is not something you worry about. A shuttle company in my town has new sprinter vans and hates them. They keep 2 7.3s and they both have over a million miles. Both have had 1 transmission replaced and 1 set of injectors. 6.0s are good if the person bullet proofing them knows what they are doing. As for pricing, no way in hell I would sell mine for 24k. Have had offers way north of that! Mine is insured for 40k through American Collectors
I wouldn't worry about mileage if the owner has really good service records. There is plenty of videos on what to look for when buying a 7.3.
Change the oil every 3-5k with Rotella T6, run Arch Oil in Diesel and after oil changes. Upgrade to a 6.0 trans cooler. replace the damn degas bottle with a billet aluminum one and drive it. I give you a few months before you starting wanting a tuner, turbo upgrades and so on. Mine will soon be getting a KC turbo and new injectors. New diesels are crazy expensive, require DEF and depending on your state modifications are a EPA fine waiting to happen. This is my opinion, I think we still get those. Haha For the price of a new truck you can purchase a old one and with the extra cash get to perform almost as good if not better. Depending on your budget.