Disclaimer: I link to products in this post using affiliate links which means I get a small credit if you purchase through those links. I link only to things I'm using or have used in the past.
I believe iPads are one of the greatest investments you can make in genealogy because they (1) let you carry your research anywhere, (2) give you more tools to organize your research, (3) allow you to store and record photos/videos, and (4) can backup to multiple remote places so your hard work is never in danger of disappearing due to a technical error.
I know, however, that an iPad is a big financial investment so I recommend that you think hard about which iPad is right for you. When I started doing genealogy notebooks on my iPad, I had a 10.5 inch iPad Pro with 64 GB of storage and it was able to hold five different genealogy notebooks for almost two years. I ran out of space, however, in part because I was also storing documents I use for work so I recently updated to an 11-inch iPad Pro with 512 GB of storage.
If you can, I'd recommend that you get as much storage as possible - you can even get an iPad with 1 TB of storage these days! Extra storage means that your iPad will run smoothly and will be able to store everything you need it to store.
I also invested in the latest Apple Pencil so I can easily fill out forms on Goodnotes by hand. (Note: different iPads are compatible with different pencils so if you're getting an older model iPad you might need the older pencil. Double-check before you purchase.) Here's a video of me unboxing my new iPad and Pencil: