I appreciate the amount of attention and comments this has received!
While I totally anticipated a lot of conflicting advice, there is also quite a bit here that is common across the board - which is what I was really looking for. Most of it aligns with what I was already recommending myself, but there have been some good alternate perspectives as well.
A little more info: they're building a new system from the ground up, so the approach is a bit different than it would be with upgrading an existing system, but they also aren't looking to just jump in quickly to rapidly throw something together. They want to buy with intent and put together a decent system even if it takes a little time - patience I wish I would have had when I first started. haha. However, I did give them a warning that it may not be realistic to hit the bullseye on the first try, so to speak.
They are not interested in vintage equipment, but happy to consider lightly used, which is helpful.
That being said, my first recommendation (which was also echoed here) was to listen to as many great complete systems as possible - either in showrooms, homes, trade shows, etc - to get a feel for some different approaches and sound qualities so they can narrow in on the 'flavor' they want to ultimately try to strive for in their own system so it isn't just shooting in the dark.
Where to start after getting that baseline idea is subject to debate (as the comment chain here has shown). Considering the space for the system is first priority, since you need to have a good idea of what you're working with. Not everyone has a dedicated room for a system, so figuring all that out first is important.
Then, I tend to lean toward starting at the ends and working inward, so to speak -- so, determining what source(s) you want to use & what speakers are preferred first and then letting those choices determine the amplification needed.
There's a lot of angles to approach this at though. For someone very green, it can be a bit overwhelming, even at a high level.