It depends on lots of things! Grading between sizes might just make it sit strangely if you use a fairly stiff fabric. Try it and see!
That cuff top has a dart which can help to shape a top and bring it in beneath the bust. Which is maybe what you mean?
When you start, it's easy to think you can just hey presto something but it's harder than it looks to do adjustments as you're saying. Eg you need to work out the right amount of ease for the curves, do you want the same amount everywhere, where does your body go in and out.
I'd go to a fabric shop and get some fabric that's nice enough but not expensive, feel it and think how it would sit, what the weight is etc.
Maybe you know this already, but proper dressmakers make a voile version first (mock up in cheap fabric) so they can check the fit and adjust before they start cutting more expensive fabric.
Really I'd say that if you want a more tailored top, find a pattern for that rather than a boxy one - but a boxy one would probably be easier to make! You can also look up how other people have made the pattern, on insta or Minerva site shows patterns and then how they've been made.
Like I said, I wouldn't approach it thinking your first attempt will be perfect. It's a fun hobby where you gain skills over time, the challenge is the enjoyable part - along the way you'll almost certainly make a load of bad stuff and make silly mistakes. For me, being able to entirely stuff it up but it doesn't matter is why it's relaxing - fucking up with no consequences and you just start again.
Some of what I've made is from tablecloths and duvet covers, often from charity shops - costs peanuts and you can practise on them. If it looks crap, in the bin it goes ;)