It’s because they launched this product with an actual reasonable amount of ram (16gb) compared to the 8gb they are still selling MacBooks with. So, if they can’t charge you $300 for a little bit of ram now, they instead are going to rape you on storage. Apple is still apple.
I don’t have any specific recommendations, and the guide from dicks seems sensible. I will say that, having used both sit-in and sit-on, I myself definitely prefer sit-in. And I’m not a pro, I mostly use mine on calm lakes and gentle streams. My only piece of advice is to not cheap out and buy some $250 piece of junk from Walmart. It’s not going to be fun and will probably turn you off of kayaking immediately. You shouldn’t have to consciously think and make an effort to stay upright, the boat should just do that on its own. I like the brand Old Town, but there are lots of good boats out there. I hope you enjoy it.