When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
AVOID:
Any 7.3 if you aren't capable of fixing it yourself. Few shops (dealers included) have anyone familiar with them.
2003-early 2004 6.0
2008-2010 6.4
2011-2014 6.7
You're basically looking for a 2004.5 - 2007 6.0L or any 6.7 after 2015.
Depending on where you live they are a bit long in the tooth by now but the 2005-07 Ford 6.0s really can be a great value for the money. You get the same front end (pretty much) that they are using now, ride is pretty good (relatively speaking), lot of reverse compatibility with new trucks all the way up to 2016 (beds, suspension parts, etc), an excellent transmission in the 5R110 (the 08-10 trucks had better transmissions but not worth dealing with a 6.4), the most dialed in version of the 6.0 and a body design that has aged very well. Even up here in the salt soaked Northeast there are still a LOT of 6.0's kicking around. There are a few specific items that need monitoring or attention with a 6.0, however those are well established by now.
AVOID:
Any 7.3 if you aren't capable of fixing it yourself. Few shops (dealers included) have anyone familiar with them.
First time I've heard anybody advise to steer clear of 7.3.
Everything else being equal (miles, condition, etc), a clean, low mileage 7.3 (with the 6spd manual!) would be at the top of my BUY list.
(yes, of course I know that the older trucks would tend to have more miles, but high mileage trucks can be found from any year. So again, I said everything else being equal)
But your point is well taken. Most diesel techs working today that take a look at a 7.3 would probably not even be able to find the DPF, EGR, SCR, DEF tank, DEF heaters, nor the myriad of sensors and miles of plumbing that they know must exist on any modern diesel.
I'd grab a 06-07 6.0 Powerstroke. Find something that has no active leaks, has good deltas, no blowby, is studded and has an older owner and then investigate further from there. For a tow pig you can't go wrong with a 6.0 and while the 7.3 is great too EVERYONE who owns one thinks 500,000mls+ with rust and garbage interior is still worth 15-20k so F that right off the hop, but the 6.0 has a fairly crap reputation still which should work to your advantage when negotiating price then again MOST people think they should be able to get back every cent they put into the truck and then some so just have to find the right sale/seller is all.