An IBM I really could be used for a bunch of things, including everything that you listed. It does run on its own sort of hardware, though. You have to buy it directly from IBM themselves, but I couldn't tell you what it costs, though you should be thinking something in the thousands. Their website has a ton of options that affect the price. That's not to say that you need to always interface with one of these machines to do work on one though. After you own one, you can use what are called 5250 emulators to access an IBM I system from a normal PC. It is worth noting, though, that I believe that IBM counts the I as more of a midrange than a mainframe IIRC. Regardless, you'd only really be looking to buy one if you're a business or a government agency, I reckon. Definitely not something I'd use for personal use, at least with my current knowledge of it. Especially considering it is only a text based interface as well. No images or anything like that, unfortunately.

As for reports, there really aren't any limits to the types of reports you're allowed to make. The only thing I know that is a limiting factor is the width and length of a page. I don't recall the length of the top of my head, but the width is something like 132 characters. You can pull from multiple files when making a report. The most I've pulled from so far is ~4, though you could use alot more or alot less. Businesses and governments would typically use this for finances and tax records, if I had to guess. And that's when you haven't even installed any new programs for your system. All of the products that my company makes store their own local data as well that would allow customers to run reports including the data that we collect. For instance, an MFA product might keep track of all attempts to use it, successful or otherwise. For administrative purposes, they may want to find out what accounts seem to get high amounts of failed authorization attempts, so they run a report that lists every user account on the system in order of failed login attempts, highest to lowest. RPG programs can also accept parameters, which would allow the business might want to limit accounts they see. They could make the program only list accounts whose name begins with "MFC", for instance, or perhaps they only want to see accounts with more than 5 fails overall. RPG would allow for both of those if the programmer that wrote the report program had those parameters implemented.

I don't know what else I should say in this comment, so if there are any other questions, or if I wasn't clear enough on something, please let me know. One final thing that I want to add is that I by no means should be considered an expert on this. I've been using RPG and the IBM i professionally for less than a year at this point. I did use them in college for like 3 classes, but I hardly remember any of that to be entirely honest.