looking at doing a Gen3 swap ???

There were some concerns with valvetrain issues in the earlier 5.7's. The 03's especially and maybe even a year or two after that had some issues with valve springs cracking. Easy enough to swap out if you're doing a rebuild anyway, but potentially an added cost to keep in mind. Everyone has different luck though. Some engines are out there with 100k+ miles with no issues and some fail in a few thousand.

As far as rebuild parts in general, the choices were pretty slim and got expensive, at least back when I did it a few years ago. There were almost no choices for rings for me, and with the dealer wanting $30 per piston for a set I was looking hard to find anything else. Ended up finding a set, but the gaps on them were pretty large. The only cam bearings available at the time as well were something like $80 a set instead of the $30 my machine shop guy was thinking they would be. Rod and crank bearings were similar, though a little more reasonably priced compared to other engines (ended up reusing mine anyway since they were in good shape). Gaskets are almost entirely O-rings, so you can reuse a lot of them if they're in good shape, but might have a harder time finding replacements if you need them.

Easy enough to rebuild the engine, nothing to unique in there compared to the older stuff. The main things to pay attention to are the rods. Most if not all the 3G Hemis use powdered metal rods, which you CAN NOT mark with a punch. Doing so will ruin them. Also, they are a cracked cap design, so each cap must go back with its respective rod and you have to be careful not to damage the rough surface when you pull them apart. The rod bolts are also torque to yield and must be replaced, but this is getting more typical in all newer engines, so it's not wildly different. Some claim that the head bolts are torque to yield as well, but I've been told by engine builders that they are not and have sucessfully reused mine so far.