Is installing a packaged HVAC unit (PTAC) a realistic DIY project?
If you're willing to blur the line between homeowner and HVAC tech, sure. If this is a split system, hooking up the linesets, purging the lines with compressed nitrogen, pumping it down, measuring the vacuum, adjusting the charge, and ensuring that the system is leak-free are all likely to be the most challenging parts. These tasks require specialized tools and knowledge. You can certainly get the tools and acquire the knowledge, but once you do that, you're basically as good as an HVAC guy, many of whom don't actually know how to do this stuff very well themselves.

Duct design is also its own huge field. If you just haphazardly run flex duct off a main trunk, you're getting a pretty typical professional residential job. But if you're doing it yourself, you have the opportunity to fabricate rigid sheet metal ducting, seal all the seams and gaps perfectly, design the entire system to be balanced, have enough returns, etc. This information would all be found in ASHRAE Manual D (D for Duct). Keep those ducts out of the attic! A crawlspace duct install is fine as long as there's a sealed vapor barrier over the dirt and the crawlspace walls are insulated.