Hey-o!
Here are two articles on the 7.3L that help explain why it’s considered one of the best Diesel engines ever made, as well as some information on why it tends to reach 300k-500k miles rather easily, if maintained obviously. The actual quote is specifically,
Robust, iron parts, conservative power, and low engine speed are the key ingredients for any diesel engine lasting forever—and if a 7.3 L has gone unmodified yet been well maintained its entire life, 400,000 to 500,000 miles is virtually guaranteed.
Then there is an article published by Truck Trend that lists the nine best “pickup truck diesel engines”, which has the following to say about the 7.3,
You could almost say this list won’t be worth a darn if the 7.3L Power Stroke engine isn’t included. Ford’s legendary powerplant, founded on the International/Navistar T44E, literally is the icon for the no-nonsense, “git’ur dunnnn” attitude that diesel engines symbolize. While the 7.3L’s entire 9-year lineage will always get much-deserved recognition, we feel the top-performing workhorse–the 275hp/525 lb-ft ’01-’03 engine (available with manual transmission)–is the best of the group for this roster.
http://www.trucktrend.com/features/1507-nine-best-diesel-engines-for-pickup-trucks
There are also a lot of forums where owners praise the engine for reaching such mileage, and beyond. Personally I know two people with 7.3L Powerstroke Diesels, one is a daily driver at 650k-ish miles and the other is a “less than frequently used” work truck around 280k-ish miles. Here is one account of a gentleman with a 7.3 at 1.3 million miles. Which yeah, you could say it’s the “odd man out”, but there are actually a number of articles easily found just like it.
Here is a funny video of a 7.3 dying just before the 800k marker, if you check the comments he actually talks about other 7.3’s he sees with high mileage still out and about.
https://www.youtube.com/watch?time_continue=8&v=HrdcKBBVTJM&feature=emb_logo
You can also say that if “well maintained” any engine “might” last that long, but from limited personal experience now, longer first hand accounts from acquaintances, and general information found on forums from other owners… “Well maintained” for these guys is apparently just an oil change every 3k miles, if anything apparently most people try to mess with the engine as little as possible because it tends to just do better if left alone. From what I’ve researched it’s not, “with regular fleet level, team of professional mechanics on hand, maintenance”, but more so just oil changes, filter changes, and other basic maintenance with occasional “mid-level” maintenance. So just the usual every day stuff, nothing over the top or outside of the average consumers ability to provide.
Being a prior Marine, I’m super picky ( cough Paranoid ~ and if you can’t tell by the number of edits on this post, also a little obsessive about rereading and double checking things. 
) about the gear I am choosing to run off into the wilds with, and so I did a great deal of research on reliability more so than anything else before choosing my vehicle. I wanted something I knew I could count on not just today, but for a solid amount of tomorrows as well. Mostly I just didn’t want some stupid small problem to leave me with a completely “dead in the water” vehicle when I’m camping in the mountains of the Yukon Territory far from all the “normal people”.
So far I’ve driven mine from West Palm to Tampa, as well as a quick trip up to Michigan and back. Ran perfectly, ironically the only “major” problem I’ve had was while it was parked in the driveway with me working on it, haha. Even then the issue was small, and quickly fixed. One thing I will say though, is it definitely seems to be one of those engines that “talks” to you and lets you know ahead of time that you’re going to be having a problem soon. I knew about a week prior I’d be calling a mechanic, so if it keeps that up in the future I’ll definitely be extremely grateful.
There is another page with literal owner submitted stats on the engine as well that I accidentally found before looking for average mileage reports, but I’m having trouble finding it again. I’ll be sure to add it if I come across it though. It was one of those sites where people go and report their mpg, mileage, and any issues they experience with the engine… It’s getting harder to find the information since Ford actually started making the 7.3L again this year.
Anyway, obviously anything can run into issues and I’m not saying it’s problem free by any means, but the 7.3 has an almost cult following due to reliability reports over the years and has garnered enough attention to have articles written on it’s reliability.
But honestly, what started off as me just wanting to find an engine that offered “above average” reliability has actually turned into a kinda cool learning experience. Truthfully, I’ve been a Dodge guy most of my life, so to be as in love with this Ford engine as I am is actually kinda funny. 
– To bring this back around to the OP Topic though… –
My advice is to go with a lower mileage engine, not because a higher mileage engine might not be as reliable, but because the lower you get it with the more you personally might be able to get out of it. Even the 7.3L, I wouldn’t have bought anything over 200k miles on it if there were options available under that, which even an older limited production year engine like the 7.3L I was still able to find under 200k. I think mine was like 160k or something, maybe 170k, but that’s the lowest I could find so that’s what I went with.