Ok, so let's be VERY clear on one additional consideration. The PSU is THE most important component in ANY build, bar none. It is literally like the butt hole of the PC hardware world. You've heard the old joke about that right? If the ***hole closes up and stops working, EVERYTHING stops working! It's exactly the same with the power supply. It doesn't matter if the rest of your hardware costs 5000 dollars. If you have a 50 dollar power supply and it stops working, none of that hardware is going to work either, or more importantly, if that cheap power supply that maybe had some shoddy build quality shorts out or it was a design that lacked some essential protections in order to cut costs, it is very possible and in fact LIKELY, because we see it ALL the time, that it takes out some of that very expensive hardware. And if you're like me, it doesn't matter if it costs 500 or 5000 dollars, if I poured some of my heard earned money into it the last damn thing in the world I want is for any of it to be exposed to slow death from a cheap power supply that is bombarding it with high levels of ripple or out of spec voltage regulation, or killing any of it because some bad soldering inside shorted out or a cap failed or it lacked the kind of protection that would have limited some other failure to just one component instead of three or four of them.
So in light of that, what is the exact model of the "unused PSU" you have? And if any or all of the power supplies you could cannibalize from another build are not known good models OR are more than a year or two old, it would be a VERY bad idea to use them with this build. It makes no sense to put 1200 dollars worth of hardware at unnecessary risk in order to save 100-ish bucks.
As far as the case goes, I've done a couple of builds in this case and while it has enough room to work in without making it a real PITA, it is also not particularly large. And the quality of it is above average while also being a fairly decent looking case. Tell me what you think of this one. It's actually SMALLER than the microATX Antec case you're looking at, at only 400x200x465mm while the Antec case is 477x220x486mm. And it is much cleaner, WITH a nice tempered glass side panel so even if you don't care about it being "ooh ahhh" it affords you the ability to easily keep an eye on or quickly peek inside to make sure everything is running normally or "is is time to blow out the dust" etc.
It's also less than HALF the price of the Antec case at only 49.99. It also has a full mesh front panel which means much better airflow and cooling than that solid front panel on the Antec case that only has some slots in the sides and maybe bottom of the front panel. That is a design that has been repeatedly shown to seriously hamper cooling performance. Not that cooling is going to be a tremendous problem with this type of build, but even so, having a full mesh front panel means any front intake fans can work much more efficiently, and thus, run slower, which also means running quieter assuming you custom profile their fan curve properly in the BIOS.
https://www.phanteks.com/Eclipse-G300A.html
PCPartPicker Part List
Case: Phanteks Eclipse G300A (1 Fan) ATX Mid Tower Case ($49.99 @ Newegg)
Total: $49.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2023-12-25 23:39 EST-0500
Now, the Antec DOES come with four fans, while the Phanteks only comes with one, but we can add three additional decent quality, fairly quiet fans and still be saving you about thirty bucks or so off the cost of what that Antec case runs. Just a thought. I'm willing to put you something together in any case you decide you like, so just get back to me on these questions and I'll poke around at what looks good within your budget right now. Obviously, if we can do it all for less than your stated budget, so much the better, and I'm pretty damn sure we can by a fair measure.