The question would be, how much do you know about the older 7.3 diesels? That age isn't turbo'd, I dont believe, so wont be as responsive. Go to "ford-trucks.com" website and find "Pre-Power Stroke Diesel (7.3L IDI & 6.9L)" page for lots of that engine specific information.
Not to be a Debbie Downer but read this guy's experience/problem on that same website page I gave you, he has the same year 7.3, here's the topic question: "Guys I need some help on this one".
Would you feel comfortable with this kind of issue and the things he's done already? If this is your first diesel, it's a steep learning curve. If you are an old hand, pardon me but I couldn't tell...
Secondly, basically ignore the mileage as ambulances usually idle for hours... The idling time is way more detrimental than miles, usually.
Lastly, how's it look inside, underneath and outside, and get it checked by a knowledgeable truck mechanic, preferably diesel tech.
The lights may have to be changed and you'll need to check on insurance and licensing prior to purchase. Look in our Misc Vehicles sub forum and search for "ambulance", a couple members have long threads about theirs.