Well, I can pick the QS200 Lite (I mean it should miss the two HDD bars, mainly) for something less than the Ace, I will think about it: and thanks for your help, Pappnaas.
Well, JFYI: I ended up picking an Aerocool QS-200 Advanced. Pretty cheap the purchase (36 bucks on Amazon, including expedited shipping), pretty cheap the case too.
Compared to a similarly sized Antek NSK-3480, it's such flimsy that you wouldn't believe it.
Any vibration is easily transmitted, so the stock fan and hard drive are much more loud than I expected, right out of the box.
With reference to the Antec, the cable management is fairly better: even if there is virtually no room behind the motherboard tray (and no cut out under the keep-out-zone, i.e. the CPU socket), the right depth let conveniently route every PSU cables under the hard drive cage, while four plastic anchors actually help quite a bit.
Layout is thus enough neat, even if working inside that small room is still a pain (to be fair, a lot less than in the quoted Antec mATX minitower).
The hard drive bar cannot quietly host mechanical hard drives, even if the 3.5" can be decoupled: so if you need more than two hard drive and a SSD, it isn't well suited, and rotational speed higher than 5400rpm should be avoided.
With reference to the max allowed dimensions for the CPU cooler, in my experience the heatsink should be no more than 160mm tall (my Cooler Master tower nearly touches the side panel), and possibly no more than 135mm wide (otherwise it would be rather difficult to fit either the CPU heatsink and the PSU).
But above all I'm a bit disappointed about the relevant thermals, its cooling prowess: even with two 120mm intakes and one 120mm exhaust, in my opinion it's not able to cool *quietly* (so taking all fans under 800rpm) anything more than a modern IGP rig, while a CPU TDP well below the 95W marks will also help a lot: at any rate, in my experience an about 80W card is enough to put that Q200 in crisis, and even a positive pressure setup didn't help.
Mind: it's being advertised as a budget gaming enclosure, but IME it can't game at all (neither quietly, nor safely).
Summarizing, probably a rather larger Fractal Define Mini would have been worked a lot better, and maybe even more cost effectively, in order to carry out a really quiet rig: as usual, there ain't no such thing as a free lunch.
I'd expect that the original Ace Ecco 250 were noticeably better with reference to vibrations, due to the sound proofing treatment which should dampen in some way those flimsy panels: on the contrary, as it has virtually no intakes, I'm really worried about the relevant cooling power (supposedly even lower than the Q200 one).