iPadOS: My main recommendation is to make sure the iPad you get can run the current version of iOS (now iPadOS). Yes, SmartGo can also run on some older versions of iOS, but it’s no secret that I’m working on an all-new version of SmartGo, and that one will require iOS 13 when it’s released. (No ETA yet.) iPads are long-lived, so you want to future-proof your purchase: at https://en.wikipedia.org/wiki/IPad#History, you can see which iPads support the latest iPadOS.
Cost: Ten years ago, the original iPad started at $499. The newest iPad is now available for only $249 at Amazon ($329 from Apple). The iPad Pro is pricier, starting at $799. Note that Apple also has refurbished iPads.
Size: The regular 9.7″ iPad was a great size, and the newest version features a 10.2″ screen. The 10.5″ iPad Air and the 11″ iPad Pro give you more screen without adding much size or weight. The screen on the 12.9″ iPad Pro is magnificent, but it’s not quite as portable as the smaller ones. With the 7.9″ iPad mini you pay more to get a smaller screen; I’d only recommend that one if portability is paramount.
Storage: 32 GB is fine for SmartGo, no need to get more. There are many other reasons to get more storage, e.g. for photos and videos, or movies for long flights, but go games and books are small and won’t impact your storage. (Can’t recommend old iPads with 16 GB, as a significant part of that is used by the system.)
Speed: For SmartGo Kifu and Go Books, speed is not currently a major factor, but I hope to take more advantage of processing power in future versions. There are apps that are based on Leela Zero or KataGo that will use all the processing power that you can give them. If that’s important to you, look at the processor: the 2018 iPad Pro with an A12X will be fastest, followed by the 2019 iPad Air (A12) and the regular 2018 iPad (A10). Note that the iPad Pro is likely due for a refresh this year.
Accessories: No accessories are needed for SmartGo. But if you want the iPad to take over more of the functions of your PC, you may want to add a keyboard cover. The newest iPad supports the Smart Keyboard, but that one requires you to perform origami each time you set it up, while the keyboard on the iPad Pro is a snap to set up. All the newest iPads support the Pencil, but the Pencil for the iPad Pro attaches and charges magnetically, which makes using it much more convenient. So if these are important, I’d recommend an iPad Pro, as it’s a significant improvement, but the difference in price is also very significant.
Hope this helps you decide on the right iPad for you. The newest iPad at $249 is a steal, and will easily pay for itself thanks to Go Books.