Long story short, build if you like doing that and have the facilities.
Buy if all you need are the speakers and don't want to wait or work at it.
Upgrade an old pair if you like to tinker.
But the takeaway is that each of these isn't going to give radically different results. Working within a price range will make all of these options come out pretty much the same.
You can buy really good speakers ready made for very little money, just like you can build a pair that will sound great for around the same cost (minus equipment and your time). And upgrading can be worth it if you keep your expectations within reason and do it properly.
There have been hundreds of thousands of pretty good sounding speakers made in the last 50 years, so buying a used set and leaving them as is certainly isn't a bad choice.
Speakers are important, but they will always sound better in a good listening space. If you are really interested in the best sound quality, you cannot ignore the room.

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