speaker design recommendation

Unless you have a fair bit of experience in crossover or analog filter design, making 3-way and in this case, 2.5-way networks can be very tricky. I won't bore you with the whys and what fors if you're not interested, however. What I would say though is that for those woofers, simply do a 2-way. They aren't extremely flexible to work as a dedicated midrange, so I would not bother with making a 2.5-way setup (in case you're wondering, anything with a .5 at the end means it's the next number up but with two identical drivers. In this case, it's a 3 way where the midrange and the woofer are the same driver. As you can imagine, this requires a flexible driver to do properly). The HDS does, however, match well with most tweeters including the one you have selected. If you're going to use it, then simply use it as the midbass in a 2-way.

For your second point, that is not an easy question to answer. Reason being, the driver's behavior depends on a lot of factors of which the enclosure volume is only one. I would vent, yes, and given their nature, I'd say around 40Hz would be fine. Those Peerless drivers in particular have fantastic low end extension, but not as much at the high end as the specs would lead you to believe. I would exploit its good low frequency behavior to its fullest and crossover both your woofer pair and your tweeter at around 2500 or 2750Hz. Fiddling with the enclosure volume and port tuning aren't the best way to ensure you'll blend the system together. Design your speakers to perform properly on their own, and I cannot think of a subwoofer which would not be able to EQed to match. Hope that helped some. )

Try some designs from Zaph|Audio.

For your idea, you should port for the woofers. How you use the woofers depends mainly on crossover. And crossover depends on how you want to use the woofers. Easiest to just use a Zaph or similar design. And if you want the simplest crossover possible then use coated paper cone woofers. Or at least avoid metal cones. Look through his woofer comparisons and see what he rates as ease of use and comments... or go and look at his graphs and look at what extends the furthest the flattest for FR and has the lowest distortion and smooth CSD while still having the right efficiency, power handling, and general build quality you need.

Otherwise you can use something like http://www.minidsp.com to build an active crossover setup, or use it or something similar (Behringer DCX2496 is the pro audio solution on the cheap). But then you need external amps and stuff. Could use an auto-eq like the Behringer DEQ2496 can do or a receiver with Audyssey MultEQ XT (wouldn't recommend less than the XT for this - look to mid-end Denon or Onkyo and I much prefer Denon), etc. Yes you can build a passive network with harder to use speakers but it gets quite difficult to do well with difficult drivers.

The challenge with using larger woofer pairs is the inevitable comb filtering that you get at upper frequencies. you'll have to do some experimenting to see if you like running both identically, or if you want to add a series inductor with the lower woofer. JBL-Harmon (and many others) have done this for decades and have plenty of reference material on the subject available for free download. The key is to just keep the lower woofer LPF first-order to avoid phase shift between the woofers.

designing a flexible crossover will make this process easier, you could even install a switch that engages this feature for quick A/B comparisons.

when building the enclosure, keep in mind that one goal is to prevent sound from the rear of the cone bouncing around inside the enclosure from coming back through the cone later. good enclosure designs address this.