Patch Base will run on any iPad that runs iOS 9 or later. Currently I think that means it could run on any iPad except the iPad 1.
That being said, if you are looking to buy an iPad to use with Patch Base, I'd recommend getting something that runs at least iOS 10, as in the years to come, iOS 9 support will eventually be dropped for newer versions of Patch Base. I strive to continue support as long as I can, but eventually it gets too difficult to support a growing number of iOS versions at the same time.
The page on Recommended MIDI Interfaces in the Patch Base Manual should point you in the right direction. Basically you just need some sort of MIDI interface for your iPad. Or, if you synth has a USB MIDI port, then you just need a USB adapter for your iPad.
Right now I don't make any promises about if or when certain editors will get made. I use people's votes to gauge the level of interest in each editor, but it also depends on how complex the editor needs to be, and how difficult it is for me to get a synth. I have to have the synth itself (at least on borrow) in order to make an editor and ensure it works well. So, for example, I know there are a lot of Ensoniq Fizmo owners out there who want me to make an editor! But I don't know anyone who has one nearby, they're rarely for sale, the MIDI documentation on them is missing/incomplete, and they cost a serious amount of cash! So please understand that I'm working within my limits, and I ultimately want to make everybody happy :)
I'm certainly thinking about it. The current issue I see is that, if I make Patch Base available for both iPad and iPhone, people will expect every editor that is available on iPad to be available on iPhone as well. And there is a significant amount of added work to make the editor interface work well on a small phone screen (imagine the 5-pages-on-iPad JD-Xi editor on a phone... it would be quite a feat!). So, I can't make Patch Base into an iPhone app without being willing to make an iPhone version of every editor I've made so far, as well as every new editor I make in the future. That is a huge commitment, and I'm not ready to make it yet.
I have also considered making synth-specific editor apps (separate apps from Patch Base) for iPhone. Still thinking on this.
These versions are less likely, at least for the time being. I'm not an Android or Windows developer, so that's a big hurdle right there. Yes, there are tools out there that "make it easier" to make an iOS app into a Windows or Android app, but it's still a very big project. Honestly, these platforms are a much lower priority for me right now.
Yes. The two apps are sold separately, and a license for one platform is only good for that platform. Not both.
No, Patch Base doesn't make any sound on its own. It serves as an editor and librarian for the actual hardware synthesizers that it supports.