It's taking me more time to go through this stuff than I have, sorry. First, congrats on your new family member! Second, thanks for the detailed reply--I now understand the spirit of your project much more.
If I understand right, you have the sub driver already and not the Iwai. Re the rest, soooooo, basically, you're going to have to measure everything and go it alone because the unit-to-unit (and lot-to-lot) variation of the cheapest drivers can be all over the place. As some who has expended a zillion hours busting my tail trying to be cheap, I can say it almost never was the right call, in-retrospect. However, gotta be pragmatic so it 100% depends on what you can source locally. If you want to take fliers (or to try to get data, these too look like (no info--speculation only)), maybe
this place can lead a path of inquiry. If you can get ME10 v1 or v2, it's fine for your "horn" experiment (as is the old dirt-cheap PRV, etc). You've got a size/package in mind and it's going to be what it's going to be. Leave extra room (more than Valeria) on sides of driver for insulation/damping/resistive material lining. Your budget and availability will box it all in. Reminds me of my favorite Arthur Ashe quote: "Start where you are, use what you have, do what you can."
My thinking is:
(1) don't be afraid of passive EQ on a constant directivity horn -- all horns are bandpass devices and this project (assuming compression drivers) will need a simple tilting up of about 6dB call it 5kHz-ish (by 6 for sure) no matter what bolt-on you find. Jockey the levels right and find the right drivers, and that's 1 cap.
(2) What you want is the smoothest response you can get in the driver+horn combo. There's a lot of ME10 data out there on several drivers (see audioxpress, among other sources) so you can see more how the driver selection changes things.
(3) You also want the midwoofer as loud as you can get it so padding is minimized (this keeps EQ simple -- whole top is, at max, 3rd-order highpass with a "lifter" cap bypassing the series leg of the pad. Getting something as loud as the Lavoce used in the Valeria keeps that possible/simple. (I like his driver choices but have issues with the cramping and the bracing in his box).
(4) Don't worry about flare types, those are more about loading and you're not really going to do / care much with 5" horns and a 6.5" driver. Personally, I don't like anything smaller than a 12 anymore but I can swallow hard for a 10 or an 8 but that's it
It's just...sure, it's all possible, it's just more work and the danger is that you don't give anything a fair try and you're out the work and the resources & time. If all you want is one finished project speaker and want it done fast chances for disappointment increase. OTOH, if it feels like Adventure and an exciting open inquiry, have a blast! Iteration will be required when the only data must be your own measurements with unknown horn+driver combinations (don't ask me how I know).
Oh--and this is personal, but, unless you know otherwise, choose aluminun + mylar (etc) instead of titanium. Some are not bothered by it--but I am and most people in my circle feel similarly. The way everybody gets drivers to play so high is to put a resonance up there--esp on the cheap titanium diaphragms. There are now more / better coatings, but look at the data in webville (esp the energy-time plots/CSD's, etc) and you'll see it if you haven't.
Should also add that, although many "pro" speakers take a lot of abuse, there's almost always someone selling some used cabs due to life changes (no more band, new kid themselves, etc). I don't know what's in your region but sometimes used JBL or EV gear (with great midwoofers!) pop-up for sale pretty cheap. Often, they have better compression drivers (or at least drivers matched to the horns they have) and pro replacement diaphragms are available commonly if they're fried.
Either way, if you can get curves, we'll all help where we can. Looks like you are on your way with the tools, though. You won't get the finest of constant directivity with those 2 horn candidates, but it'll be close-enough above 2k2 or so from teh looks of it. That big PRV would get you closer to 1k2 if you can make a "normal ewave" (like that marantz-o-wave in the link).
Do your best to say no to 80-anything dB/W, though. 9X or bust.
FWIW. Keep us posted.
