Build those items as your 2nd item if you really need it early, if not, you can delay it to 3rd, 4th etc. depending on when you need it. Getting Luden's Echo or Blackfire Torch as your 1st item is practically a requirement, you are not a champion without it.

As your 2nd item you have all the variability you need. Do you need a way to block ultimates and cover for AD? Go Zhonya. Do you need to get MR and defend against AP burst? Go Banshee. If situational items into specific matchups don't apply? Continue with the usual build and try to strive for Rabadon's Deathcap as your 3rd item & Void Staff as your 4th depending on the MR stacking of the enemy team. Basically, if you aren't in direct need of situational items for the game in question, you want to maximize your damage by building the regular build.

As for how many, I would say yes, it's fine to build how many you need, but I would say don't go above 2, this is cuz rabadon, void, sorc shoes & ludens are always going to be mandatory in most games, especially if they stall out for long times. But it really depends on the game, for example if I am in a game where I needed a Morellonomicon to counter a Soraka on bot or a Dr. Mundo on top, and I needed a Banshee's Veil to counter my opponent, let's say it's a Fizz or LeBlanc. Then I would build Oblivion Orb and sit on it, then get banshee and continue the build, i would only get morello as my last item when it's all I have left to buy, because you don't need it fully upgraded so early. My build would then be for example: Luden's Echo > Oblivion Orb/ Banshee's Veil > Sorcerer's Shoes > Rabadon's Deathcap > Void Staff > Morellonomicon

Boots of Swiftness can be good in certain situations, like against heavy slows (e.g. Ashe, Nasus, Anivia) or if you just need extra map mobility, which is more than likely why it's being used quite a lot on her this time around. But Sorcerer's Shoes are almost always better when you're not in these situations because they directly amplify your burst. I'd say Swifties have niche value, and whilst Sorc's are almost always the optimal choice, there are definitely times where you can pick the others instead, for sure.

I'm glad to know you enjoyed it. 😄