I actually did quite a bit of research on this a few weeks ago because I was looking at lighting solutions. The problem with all the "projector" housings out there (especially the one you listed) is that they aren't true projector lenses. They're still made for the Halogen H4 style bulb, and many people just choose to throw an HID bulb into them. While this is better than putting a HID bulb into a more traditional housing that lacks this "projector style" lens as the light is much less likely to simply refract all over the place it's still not idea.
That being said, there are some interesting articles on the internet of people who have put true projector lenses into out 7" round headlights. This is due to the fact that Jeep Wranglers, Mini Coopers, and a few other popular vehicles share the same style of headlight. There's even a few people online that will do a true retrofit and conversion for you.
If you HAVE to go with one of these other Seudo-Projector style housings, there are some things you want to look for. You want them to be SAE and DOT approved. Many of these china based knock-offs don't carry these certifications, and you'll see it in the quality. You can also get a nice fix it ticket if you get pulled over and the cop wants to be a real stickler (very unlikely, but I've had a few friends experience this) You'll know that they're approved because they'll be listed in the product description. Gaining these approvals is an expensive process so they're usually proud of it and will throw it out there.
As far as HIDs go, 4300K is still considered a WARM temperature, meaning that there will be hints of yellow in it. 5000K is the highest light output, it's about as pure white as you can go without any blue tinting. 6000K tends to be PERCEIVED as brighter, but it's not, it has just a little bit of blue tinting too it. 5000k and 6000k are incredibly common in the HID world, replacement bulbs are easy to get, and prices are low. Stay away from 8000, 10000, and 120000, it's a Mustang for goodness sake show some respect, not to mention some of these highly colored lights are actually illegal in some states (your burden to check local legislation)
You want to make sure you get a Bi-Lamp HID kit, so that you can retain your High/Low beam operation. Many of the cheaper kits don't have a High beam option. For HIDs this is typically achieved with a small solenoid that physically moved the bulb forwards and backwards changing the focal point within the elliptical shape of the headlight housing.
Then there's 55W and 65W units, slim or normal ballasts, and a host of other options. Also remember that HIDs will almost always require you to bypass the headlight circuit to power them directly from the battery, so you'll want to fuse them yourselves. (A nice benefit though is this reduces the loading on the headlight circuit and can cause your dash lights to increase in brightness!)
An alternative to HID lights, is there are some companies offering LED 7" headlights. The one that I'm looking to use is KC Daylighter. Keep in mind, these are at the moment, quite expensive, and there's no guarantees they'll even fit, but you've got headlights for life without the need to replace. (Here's a link to the LED headlights:
https://www.kchilites.com/lights/headlights/7-led-headlight-kit PRICEY!)
At the end of the day, it's your car, do what you like! But keep in mind that you don't want to blind others with poorly done headlights.
