Questions I have about iPads or tablets - do you know the answers?

I am thinking of getting my dad a tablet to play with (he's an elderly but technologically competent nerd, though is a bit puzzled by touchscreens).

My main enthusiasm for the idea of these devices (I don't have one myself but have an iPhone) is the instancy with which they can be switched on and used, no waiting for it to boot up (unless you do a hard shut down) and the fact that he'd be able to send and receive emails with it as well.

My mental model of tablets is that they're basically identical to my iPhone except that they can't make telephone calls (other than Skype of course) and are useless outside the home unless they have a 3G or similar connection.

With my level of understanding established these are my questions about them, but if I've missed a question I should have asked please suggest it, along with an answer if possible.

• Is it only iPads that have 3G or 4G capability, not generic £79 tablets from Argos? Are they all, pretty much by definition, wifi-enabled? I suppose if it's something that doesn't have to be useless if taken outside the home then that might be a plus so I would consider 3G if available.
• If we get a 3G-ready one is it possible to do it as a pay-as-you-go thing rather than a contract thing, and what's the 'distance' between payments. Eg if he wants to use it outside the house in May and then not again until August is that possible or does he have to pay even if unused. I suppose what I want is the equivalent of the 'dongle' device that I can buy for £30 and plug into a USB port on my laptop and 'get internet' for £5/day but months can go by without me using it, at no extra cost. I just pay for what I use. 
• Presumably the fact that it can't make telephone calls (bit awkward to hold to the ear though increasingly I see people holding their phones in what I think is an unnatural way anyway) is offset by the fact that they can do Skype via voice-over-internet-protocol (VOIP) or whatever technology they're currently using?
• How easy is the touchscreen and on-screen keyboard to use, particularly for older arthritic fingers and eyesight that's not terrible but not awfully keen. Apple seem very good at making products that are easily usable by a variety of folk but are cheaper generic tablets also perfectly adequate at letting you change the size of the text etc? 
• I assume the wifi connection will let him use the internet, get email, watch YouTube videos and general stuff that he also gets on his regular computer.
• If he can't get to grips with the on-screen keyboard how easy is it it to attach an external keyboard?
• Do iPads (or any tablet) have USB ports?
• He'd be able to take a photograph of something and email it to me wouldn't he? Some iPads have two cameras. To be honest I was a bit surprised to find that my iPhone has two cameras.
• Can he download apps like Kindle and stuff from the app store? Presumably he can't get the App Store from Apple on a non-Apple product but what are the apps like on generic tablets?
• I can't imagine he'd fill it with stuff so I assume the lower 16GB is sufficient but happy to hear otherwise. 
• I think I'd probably go for a large thing, I really like the iPad version 1 or 2, quite like the mini but I prefer the bigger screen (though a slightly smaller one might be easier for him to handle).

Thanks :)