Basically the Mini is smallest, the base iPad is best value but has an older design and lesser (though still perfectly decent) display, the new Air shares the Pro's design but lacks some bells and whistles, the Pro offers a few things that some people will tell you are essential and others will shrug at.
For many people the regular iPad will do everything they want for a lot less than the Air and half the price of the Pro. Others will have requirements that are only met by a different model. For some that extra cost may be worth it just to have something "nicer", but for others it would need a tangible benefit to justify it. Without knowing what you intend to use it for or your budgetary preferences/constraints are it's impossible to make any recommendation.
They are all Pencil compatible these days, but there are 2 models of Pencil. The iPads that use a Lightning port (Mini and basic iPad) are compatible with the original Pencil, those that use a USB-C port (Pro and the new Air) are compatible with Pencil 2, which has some extra features, doesn't have the awkward "pull off a little cap and plug it into the Lightning port" charging method, and costs even more (the Pencils are rather expensive for what they are, but that pretty much goes without saying. They work well). Rather than try to provide a comprehensive list of accessories I suggest you go to the Apple Store website, select accessories, specify which iPad you want them to be compatible with and see what the options are for each. You'll find that more expensive iPads generally have more expensive accessories as well, so again if budget is a factor and you have a list of accessories you want that may be worth factoring into your choice. Of course there are also third party accessories which will cost less than anything you'll find in the Apple Store.
One tip I can give: try to understand your needs before making a choice. The product tiers are designed in order to upsell you, often beyond what you need, and I see that working quite often in this forum. For example, someone might think the Air 4 suits them fine, but be unsure whether the 64GB storage of the base model is enough. The next option is 256GB, but that adds £150 to the price. And then the base 11" Pro only costs £40 more than that, has 128GB storage and some of the "bells and whistles" I referred to earlier, and a chorus of voices will tell them that one or other of these is "really nice" and woth the extra £40... and before you know it someone who had thought that a £579 Air 4 was probably right for them is paying £769 for an 11" Pro. And since it was only the storage that they were unsure of, they've effectively paid £190 for an extra 64GB. So as I say, try to understand what you actually need before making your choice, and beware of the siren cry of "but it's only XXX more for...", because that's stepping on an escalator.
