I can only answer these questions, but I'm sure others can chine in.
2. Is it possible, somewhere on the internet, to buy a hardware version of Dectalk with version 4.2 or 4.3, that can work with a typical Windows 7 laptop, and what would it cost?
You mean the DECtalk Express? Those are pretty hard to come by these days on auction sites. I once saw a few listings on eBay, but they're gone now. Even if you DID manage to find one, you'd never know if it had version 4.2 or 4.61, since the newer DECtalk Express units shipped with Force Computers' DECtalk version 4.61, even though it is possible to downgrade the firmware back to version 4.2. Plus, you'd need a Serial to USB cable, since the DECtalk Express was meant to work with old PCs that have a Serial port. But, it does work on modern Windows, as I've read reports of users getting it to work under Windows 8 with JAWS.
3. Is it possible to get a working software or hardware version of Stephen Hawking's Speech Plus Prose-2000 voice?
There was an emulator version done recently, made to work on the Raspberry Pi, but as it stands now, we may never have access to it, only his family and the team who made it have access to it, even though there's an implementation built into MAME that doesn't even work. That said, the closest thing you'll get is using TruVoice.
4. I hear samples of the Microsoft Whistler system with Mike and Mary from the Dectalk Archive, but is it still possible to get it work on Windows 7?
That project was shelved before it was even released by Microsoft. Had it been released, it would've been a competitor of sorts to VocalWriter, even though there are rumors floating around that you can find it on the CD of certain Windows 2000 and XP distributions. Take that info with a grain of salt though.
Hello. He-he-he-he-have some respect and don't spoil the game. It's impossible to have mysteries nowadays, because of nosy people like you. Please keep all of this between us. If you post it online, I won't make any more secrets. No one will be impressed. It will be your fault! He-he-he-he-ha-he-he-ha-he-he.
- abc_123_a.ogg, Undertale v1.000 -